feat(stdlib): add Anim.* + Tween.* — deterministic 2D animation & tweening (#5)

Two ECS-native, deterministic namespaces for 2D motion, driven off the fixed
frame clock so replays and lockstep netcode reproduce every frame and every
eased value exactly. Both are pure computed-inline Q16.16 / integer math (no new
runtime, no heap) — the game stores a timer on a component and calls these each
frame, exactly the way Collision.* / Grid.* are used.

Anim.* — spritesheet frame animation:
  - Anim.frame(timer,fps,count) -> int      looping frame index
  - Anim.once(timer,fps,count) -> int       one-shot, clamps on the last frame
  - Anim.pingpong(timer,fps,count) -> int   bounce 0..count-1..0
  - Anim.finished(timer,fps,count) -> bool   has a one-shot run past its end?
  - Anim.duration(fps,count) -> fixed        seconds for one cycle
  - Anim.cell_x/cell_y(frame,cols,cell) -> int  source rect on a grid sheet

Tween.* — value interpolation over a timeline:
  - Tween.progress/loop/yoyo(timer,duration) -> fixed  normalized amount
  - Tween.done(timer,duration) -> bool
  - Tween.ease(t, mode) -> fixed             shape by a literal curve 0..6,
                                             the same curves as Ease.* (now
                                             factored into a shared ease_eval)
  - Tween.number/round/point/tint(from,to,t) blend a fixed / int / Vector / color

The typed blends reuse the existing fixed / Vector / color helpers, and
Tween.ease shares Ease.*'s exact formulas via the new ease_eval(mode,t) — one
source of truth for every easing curve in the engine.

examples/library/anim.ludic asserts 34 cases (frame math, clamping, ping-pong,
cell geometry, timeline clamp/loop/yoyo, rounding, color/vector blends, and
Ease.in == Tween.ease(.,1)); wired into x test (now 62 passed). Docs: Anim +
Tween sections with 16 per-symbol pages, inventory/coverage green. Seed
reseeded; the C-free bootstrap fixpoint holds.

The stateful sugar the proposal sketches (named clips, Anim.play, fluent
Tween.chain/parallel handles, and an auto-injected advance system) is deliberately
left as a follow-up — this lands the deterministic math core both halves stand on.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/anim/_section.md

@@ -0,0 +1,7 @@
+---
+id: anim
+title: Anim
+order: 28
+---
+
+Spritesheet frame animation off the fixed frame clock. Store an elapsed <code>timer</code> (seconds, a <code>fixed</code>) on a component and each frame ask <a href="anim-frame"><code>Anim.frame</code></a> / <a href="anim-once"><code>Anim.once</code></a> / <a href="anim-pingpong"><code>Anim.pingpong</code></a> which cell to draw; <a href="anim-cell_x"><code>Anim.cell_x</code></a>/<a href="anim-cell_y"><code>Anim.cell_y</code></a> turn a frame index into a source rectangle on the sheet. Everything is integer/fixed and deterministic — the same timer reproduces the same frame every run, so replays and lockstep netcode match exactly.

docs/language/anim/anim-cell_x.md

@@ -0,0 +1,31 @@
+---
+id: anim-cell_x
+name: Anim.cell_x
+category: anim
+kind: namespace-method
+tokens: Anim.cell_x
+sig: Anim.cell_x(frame, cols, cell_w) -> int
+tip: The source x (pixels) of a frame on a grid spritesheet.
+order: 6
+ns: Anim
+member: cell_x
+---
+
+Turns a frame index into the left pixel of its cell on a spritesheet laid out as a grid of <code>cols</code> columns: <code>(frame mod cols) * cell_w</code>. Combine with <a href="anim-cell_y"><code>Anim.cell_y</code></a> to get the top-left source coordinate to blit from.
+
+Parameters:
+- `frame` — the frame index (e.g. from <a href="anim-frame"><code>Anim.frame</code></a>)
+- `cols` — columns in the sheet
+- `cell_w` — cell width in pixels
+
+```ludic
+program Demo {
+  handler Run phase Update {
+    let f  = Anim.frame(fixed(1), 12, 8)
+    let sx = Anim.cell_x(f, 4, 16)
+    let sy = Anim.cell_y(f, 4, 16)
+    print(sx)
+    print(sy)
+  }
+}
+```

docs/language/anim/anim-cell_y.md

@@ -0,0 +1,29 @@
+---
+id: anim-cell_y
+name: Anim.cell_y
+category: anim
+kind: namespace-method
+tokens: Anim.cell_y
+sig: Anim.cell_y(frame, cols, cell_h) -> int
+tip: The source y (pixels) of a frame on a grid spritesheet.
+order: 7
+ns: Anim
+member: cell_y
+---
+
+Turns a frame index into the top pixel of its cell on a spritesheet laid out as a grid of <code>cols</code> columns: <code>(frame / cols) * cell_h</code>. Pair with <a href="anim-cell_x"><code>Anim.cell_x</code></a> for the full source rectangle of the frame.
+
+Parameters:
+- `frame` — the frame index (e.g. from <a href="anim-frame"><code>Anim.frame</code></a>)
+- `cols` — columns in the sheet
+- `cell_h` — cell height in pixels
+
+```ludic
+program Demo {
+  handler Run phase Update {
+    let f  = Anim.frame(fixed(1), 12, 8)
+    let sy = Anim.cell_y(f, 4, 16)
+    print(sy)
+  }
+}
+```

docs/language/anim/anim-duration.md

@@ -0,0 +1,27 @@
+---
+id: anim-duration
+name: Anim.duration
+category: anim
+kind: namespace-method
+tokens: Anim.duration
+sig: Anim.duration(fps, count) -> fixed
+tip: Seconds for one full cycle of a clip: count / fps.
+order: 5
+ns: Anim
+member: duration
+---
+
+Returns the length of one cycle of a clip in seconds as a <code>fixed</code>: <code>count / fps</code>. Handy to schedule the next event, size a progress bar, or line a tween up with an animation.
+
+Parameters:
+- `fps` — frames per second
+- `count` — number of frames in the clip
+
+```ludic
+program Demo {
+  handler Run phase Update {
+    let secs = Anim.duration(12, 4)   # 4 frames / 12 fps = 0.333s
+    print(secs)
+  }
+}
+```

docs/language/anim/anim-finished.md

@@ -0,0 +1,30 @@
+---
+id: anim-finished
+name: Anim.finished
+category: anim
+kind: namespace-method
+tokens: Anim.finished
+sig: Anim.finished(timer, fps, count) -> bool
+tip: True once a one-shot clip has run past its last frame.
+order: 4
+ns: Anim
+member: finished
+---
+
+Returns <code>true</code> once <code>floor(timer * fps) >= count</code> — i.e. a one-shot clip driven by <a href="anim-once"><code>Anim.once</code></a> has played its final frame. Use it to despawn an effect, fire a follow-up, or switch back to an idle clip.
+
+Parameters:
+- `timer` — elapsed seconds (a `fixed`)
+- `fps` — frames per second
+- `count` — number of frames in the clip
+
+```ludic
+program Demo {
+  property Blast { timer: fixed = 0.0 }
+  model Boom { Blast }
+  handler Run phase Update {
+    Blast.timer = Blast.timer + Time.delta()
+    if Anim.finished(Blast.timer, 15, 6) { despawn(self()) }
+  }
+}
+```

docs/language/anim/anim-frame.md

@@ -0,0 +1,31 @@
+---
+id: anim-frame
+name: Anim.frame
+category: anim
+kind: namespace-method
+tokens: Anim.frame
+sig: Anim.frame(timer, fps, count) -> int
+tip: The looping frame index for an elapsed timer at a given fps.
+order: 1
+ns: Anim
+member: frame
+---
+
+Returns the current frame of a looping clip: <code>floor(timer * fps)</code> reduced modulo <code>count</code>. <code>timer</code> is elapsed seconds as a <code>fixed</code>, <code>fps</code> the clip's frames per second, and <code>count</code> the number of frames. The clip wraps forever — frame <code>count-1</code> is followed by frame <code>0</code>.
+
+Parameters:
+- `timer` — elapsed seconds (a `fixed`)
+- `fps` — frames per second
+- `count` — number of frames in the clip
+
+```ludic
+program Demo {
+  property Sprite { timer: fixed = 0.0 }
+  model Hero { Sprite }
+  handler Run phase Update {
+    Sprite.timer = Sprite.timer + Time.delta()
+    let cell = Anim.frame(Sprite.timer, 12, 4)   # 4-frame run at 12 fps
+    print(cell)
+  }
+}
+```

docs/language/anim/anim-once.md

@@ -0,0 +1,31 @@
+---
+id: anim-once
+name: Anim.once
+category: anim
+kind: namespace-method
+tokens: Anim.once
+sig: Anim.once(timer, fps, count) -> int
+tip: A non-looping frame index that clamps on the last frame.
+order: 2
+ns: Anim
+member: once
+---
+
+Like <a href="anim-frame"><code>Anim.frame</code></a> but for a one-shot clip: the frame is <code>min(floor(timer * fps), count - 1)</code>, so once it reaches the last frame it stays there instead of wrapping. Use it for a play-once animation like an explosion or a door opening; pair it with <a href="anim-finished"><code>Anim.finished</code></a> to know when it is done.
+
+Parameters:
+- `timer` — elapsed seconds (a `fixed`)
+- `fps` — frames per second
+- `count` — number of frames in the clip
+
+```ludic
+program Demo {
+  property Blast { timer: fixed = 0.0 }
+  model Boom { Blast }
+  handler Run phase Update {
+    Blast.timer = Blast.timer + Time.delta()
+    let cell = Anim.once(Blast.timer, 15, 6)
+    print(cell)
+  }
+}
+```

docs/language/anim/anim-pingpong.md

@@ -0,0 +1,31 @@
+---
+id: anim-pingpong
+name: Anim.pingpong
+category: anim
+kind: namespace-method
+tokens: Anim.pingpong
+sig: Anim.pingpong(timer, fps, count) -> int
+tip: A frame index that bounces 0..count-1..0 and repeats.
+order: 3
+ns: Anim
+member: pingpong
+---
+
+Returns a frame index that plays forward to <code>count-1</code>, then back to <code>0</code>, then forward again — a triangle wave over the frames. Ideal for a two-way idle bob or a breathing/pulsing loop where a plain wrap would snap.
+
+Parameters:
+- `timer` — elapsed seconds (a `fixed`)
+- `fps` — frames per second
+- `count` — number of frames in the clip
+
+```ludic
+program Demo {
+  property Idle { timer: fixed = 0.0 }
+  model Fish { Idle }
+  handler Run phase Update {
+    Idle.timer = Idle.timer + Time.delta()
+    let cell = Anim.pingpong(Idle.timer, 8, 4)   # 0 1 2 3 2 1 0 1 ...
+    print(cell)
+  }
+}
+```

docs/language/tween/_section.md

@@ -0,0 +1,7 @@
+---
+id: tween
+title: Tween
+order: 29
+---
+
+Value interpolation over a timeline, off the fixed frame clock. The timeline helpers <a href="tween-progress"><code>Tween.progress</code></a>/<a href="tween-loop"><code>Tween.loop</code></a>/<a href="tween-yoyo"><code>Tween.yoyo</code></a> turn an elapsed <code>timer</code> and a <code>duration</code> into a normalized amount; <a href="tween-ease"><code>Tween.ease</code></a> shapes that amount through an easing curve (shared with <a href="ease"><code>Ease</code></a>); and the typed blends <a href="tween-number"><code>Tween.number</code></a>/<a href="tween-round"><code>Tween.round</code></a>/<a href="tween-point"><code>Tween.point</code></a>/<a href="tween-tint"><code>Tween.tint</code></a> interpolate a <code>fixed</code>, <code>int</code>, <code>Vector</code>, or color. All deterministic fixed-point, so a replay reproduces every eased value exactly.

docs/language/tween/tween-done.md

@@ -0,0 +1,29 @@
+---
+id: tween-done
+name: Tween.done
+category: tween
+kind: namespace-method
+tokens: Tween.done
+sig: Tween.done(timer, duration) -> bool
+tip: True once a one-shot tween's timer reaches its duration.
+order: 4
+ns: Tween
+member: done
+---
+
+Returns <code>true</code> once <code>timer >= duration</code> — the companion to <a href="tween-progress"><code>Tween.progress</code></a> for ending a one-shot tween: kick off the next step, clear a component, or snap the value to its target.
+
+Parameters:
+- `timer` — elapsed seconds (a `fixed`)
+- `duration` — the tween's length in seconds (a `fixed`)
+
+```ludic
+program Demo {
+  property Slide { timer: fixed = 0.0 }
+  model Menu { Slide }
+  handler Run phase Update {
+    Slide.timer = Slide.timer + Time.delta()
+    if Tween.done(Slide.timer, fixed(1)) { print(1) }
+  }
+}
+```

docs/language/tween/tween-ease.md

@@ -0,0 +1,37 @@
+---
+id: tween-ease
+name: Tween.ease
+category: tween
+kind: namespace-method
+tokens: Tween.ease
+sig: Tween.ease(t, mode) -> fixed
+tip: Shape a 0..1 amount through an easing curve chosen by a literal mode.
+order: 5
+ns: Tween
+member: ease
+---
+
+Shapes a normalized amount <code>t</code> (<code>0.0</code>..<code>1.0</code>) through one of the engine's easing curves and returns the eased <code>fixed</code>. <code>mode</code> must be a literal integer selecting the curve — the same curves as the <a href="ease"><code>Ease</code></a> namespace:
+
+- `0` — linear
+- `1` — ease-in (quadratic)
+- `2` — ease-out (quadratic)
+- `3` — smooth ease-in-out
+- `4` — ease-in-back (overshoots below 0)
+- `5` — ease-out-elastic (springy settle)
+- `6` — ease-out-bounce
+
+Parameters:
+- `t` — the amount to shape (a `fixed` in 0..1, e.g. from <a href="tween-progress"><code>Tween.progress</code></a>)
+- `mode` — a literal `int` 0..6 selecting the curve
+
+```ludic
+program Demo {
+  handler Run phase Update {
+    let t = Tween.progress(fixed(1), fixed(2))
+    let e = Tween.ease(t, 4)          # ease-in-back
+    let x = Tween.round(0, 200, e)
+    print(x)
+  }
+}
+```

docs/language/tween/tween-loop.md

@@ -0,0 +1,30 @@
+---
+id: tween-loop
+name: Tween.loop
+category: tween
+kind: namespace-method
+tokens: Tween.loop
+sig: Tween.loop(timer, duration) -> fixed
+tip: A repeating 0..1 sawtooth amount over duration.
+order: 2
+ns: Tween
+member: loop
+---
+
+Returns the fractional part of <code>timer / duration</code> — a sawtooth that ramps <code>0.0</code>→<code>1.0</code> then jumps back to <code>0.0</code> and repeats. Use it for a continuously cycling value: a scrolling offset, a rotating hue, a repeating pulse.
+
+Parameters:
+- `timer` — elapsed seconds (a `fixed`)
+- `duration` — seconds per cycle (a `fixed`)
+
+```ludic
+program Demo {
+  property Spin { timer: fixed = 0.0 }
+  model Coin { Spin }
+  handler Run phase Update {
+    Spin.timer = Spin.timer + Time.delta()
+    let t = Tween.loop(Spin.timer, fixed(2))   # a 2-second cycle
+    print(t)
+  }
+}
+```

docs/language/tween/tween-number.md

@@ -0,0 +1,29 @@
+---
+id: tween-number
+name: Tween.number
+category: tween
+kind: namespace-method
+tokens: Tween.number
+sig: Tween.number(from, to, t) -> fixed
+tip: Linear blend of two fixeds by amount t.
+order: 6
+ns: Tween
+member: number
+---
+
+Interpolates two <code>fixed</code> values: <code>from + (to - from) * t</code>. Feed it an eased <code>t</code> from <a href="tween-ease"><code>Tween.ease</code></a> to tween any fractional quantity — an alpha, a scale, a fixed-point position.
+
+Parameters:
+- `from` — the value at `t = 0` (a `fixed`)
+- `to` — the value at `t = 1` (a `fixed`)
+- `t` — the blend amount (a `fixed`, usually 0..1)
+
+```ludic
+program Demo {
+  handler Run phase Update {
+    let t = Tween.progress(fixed(1), fixed(2))
+    let scale = Tween.number(fixed(1), fixed(2), Tween.ease(t, 2))
+    print(scale)
+  }
+}
+```

docs/language/tween/tween-point.md

@@ -0,0 +1,31 @@
+---
+id: tween-point
+name: Tween.point
+category: tween
+kind: namespace-method
+tokens: Tween.point
+sig: Tween.point(from, to, t) -> Vector
+tip: Component-wise blend of two Vectors by amount t.
+order: 8
+ns: Tween
+member: point
+---
+
+Interpolates two <a href="vector"><code>Vector</code></a> endpoints component-wise by amount <code>t</code> — the 2D version of <a href="tween-number"><code>Tween.number</code></a>. Move an entity along a straight path, or ease a camera toward a target, in one call.
+
+Parameters:
+- `from` — the point at `t = 0` (a `Vector`)
+- `to` — the point at `t = 1` (a `Vector`)
+- `t` — the blend amount (a `fixed`, usually 0..1)
+
+```ludic
+program Demo {
+  handler Run phase Update {
+    let a = Vector.make(fixed(0), fixed(0))
+    let b = Vector.make(fixed(100), fixed(40))
+    let t = Tween.progress(fixed(1), fixed(2))
+    let p = Tween.point(a, b, Tween.ease(t, 3))
+    print(Vector.x(p))
+  }
+}
+```

docs/language/tween/tween-progress.md

@@ -0,0 +1,31 @@
+---
+id: tween-progress
+name: Tween.progress
+category: tween
+kind: namespace-method
+tokens: Tween.progress
+sig: Tween.progress(timer, duration) -> fixed
+tip: A one-shot 0..1 amount, clamped, for timer over duration.
+order: 1
+ns: Tween
+member: progress
+---
+
+Returns <code>timer / duration</code> clamped to <code>0.0</code>..<code>1.0</code> — the normalized progress of a one-shot tween. Feed it to <a href="tween-ease"><code>Tween.ease</code></a> to shape it, then to a typed blend. Once <code>timer</code> reaches <code>duration</code> it pins at <code>1.0</code> (a non-positive <code>duration</code> also reads as done).
+
+Parameters:
+- `timer` — elapsed seconds (a `fixed`)
+- `duration` — the tween's length in seconds (a `fixed`)
+
+```ludic
+program Demo {
+  property Fade { timer: fixed = 0.0 }
+  model Panel { Fade }
+  handler Run phase Update {
+    Fade.timer = Fade.timer + Time.delta()
+    let t = Tween.progress(Fade.timer, fixed(1))
+    let a = Tween.round(0, 255, Tween.ease(t, 2))
+    print(a)
+  }
+}
+```

docs/language/tween/tween-round.md

@@ -0,0 +1,32 @@
+---
+id: tween-round
+name: Tween.round
+category: tween
+kind: namespace-method
+tokens: Tween.round
+sig: Tween.round(from, to, t) -> int
+tip: Linear blend of two ints by amount t, rounded to the nearest int.
+order: 7
+ns: Tween
+member: round
+---
+
+Interpolates two <code>int</code> endpoints by amount <code>t</code> and rounds to the nearest integer (half up): <code>from + round((to - from) * t)</code>. The go-to for tweening a pixel position, a channel byte, or any integer field. Combine with <a href="tween-ease"><code>Tween.ease</code></a> for shaped motion.
+
+Parameters:
+- `from` — the value at `t = 0` (an `int`)
+- `to` — the value at `t = 1` (an `int`)
+- `t` — the blend amount (a `fixed`, usually 0..1)
+
+```ludic
+program Demo {
+  property Slide { timer: fixed = 0.0 }
+  model Card { Slide }
+  handler Run phase Update {
+    Slide.timer = Slide.timer + Time.delta()
+    let t = Tween.progress(Slide.timer, fixed(1))
+    let x = Tween.round(0, 320, Tween.ease(t, 2))
+    print(x)
+  }
+}
+```

docs/language/tween/tween-tint.md

@@ -0,0 +1,29 @@
+---
+id: tween-tint
+name: Tween.tint
+category: tween
+kind: namespace-method
+tokens: Tween.tint
+sig: Tween.tint(from, to, t) -> int
+tip: Per-channel blend of two colors by amount t.
+order: 9
+ns: Tween
+member: tint
+---
+
+Blends two colors (<code>0x00RRGGBB</code> ints) channel by channel by amount <code>t</code> — the color equivalent of <a href="tween-number"><code>Tween.number</code></a>. Fade a sprite to white on hit, cross-fade a sky, or pulse a UI accent. Matches <a href="color-lerp"><code>Color.lerp</code></a>, ready for an eased <code>t</code> from <a href="tween-ease"><code>Tween.ease</code></a>.
+
+Parameters:
+- `from` — the color at `t = 0` (an `int`, `0x00RRGGBB`)
+- `to` — the color at `t = 1` (an `int`, `0x00RRGGBB`)
+- `t` — the blend amount (a `fixed`, usually 0..1)
+
+```ludic
+program Demo {
+  handler Run phase Update {
+    let t = Tween.progress(fixed(1), fixed(2))
+    let c = Tween.tint(Color.rgb(0, 0, 0), Color.rgb(255, 255, 255), t)
+    print(c)
+  }
+}
+```

docs/language/tween/tween-yoyo.md

@@ -0,0 +1,31 @@
+---
+id: tween-yoyo
+name: Tween.yoyo
+category: tween
+kind: namespace-method
+tokens: Tween.yoyo
+sig: Tween.yoyo(timer, duration) -> fixed
+tip: A repeating 0..1..0 triangle amount over duration.
+order: 3
+ns: Tween
+member: yoyo
+---
+
+Returns a triangle wave: <code>timer / duration</code> ramps <code>0.0</code>→<code>1.0</code> over the first <code>duration</code>, then <code>1.0</code>→<code>0.0</code> over the next, and repeats. The smooth back-and-forth for a hover bob, a pulsing highlight, or a breathing scale — no snap at the ends the way <a href="tween-loop"><code>Tween.loop</code></a> has.
+
+Parameters:
+- `timer` — elapsed seconds (a `fixed`)
+- `duration` — seconds for each leg (a `fixed`)
+
+```ludic
+program Demo {
+  property Bob { timer: fixed = 0.0 }
+  model Pickup { Bob }
+  handler Run phase Update {
+    Bob.timer = Bob.timer + Time.delta()
+    let t = Tween.yoyo(Bob.timer, fixed(1))
+    let y = Tween.round(100, 108, Tween.ease(t, 3))
+    print(y)
+  }
+}
+```

examples/library/anim.ludic

@@ -0,0 +1,87 @@
+# anim.ludic — Anim.* spritesheet frame animation and Tween.* value tweening,
+# both pure and deterministic off the fixed frame clock. Each assertion that
+# holds prints its number, so a full run prints:
+#   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
+# A `timer` is elapsed seconds as a fixed; fps / frame counts are ints; tween
+# amounts are a fixed in 0..1. Timers are exact-in-fixed so floor(timer*fps)
+# never lands ambiguously on a frame boundary.
+program Anim {
+  property Tag { v: int = 0 }
+  model Marker { Tag }
+
+  handler Boot phase Start {
+    let F0   = fixed(0)
+    let F1   = fixed(1)
+    let HALF = fixed(1) / fixed(2)        # 0.5
+    let QTR  = fixed(1) / fixed(4)        # 0.25
+    let E18  = fixed(1) / fixed(8)        # 0.125
+    let E78  = fixed(7) / fixed(8)        # 0.875
+    let F125 = fixed(5) / fixed(4)        # 1.25
+    let F15  = fixed(3) / fixed(2)        # 1.5
+    let TWO  = fixed(2)
+    let FOUR = fixed(4)
+    let WHITE = 16777215                  # 0xFFFFFF
+    let MID   = 8355711                   # 0x7F7F7F
+
+    # --- Anim.frame: looping frame index at 8 fps over a 4-frame clip ---
+    if Anim.frame(F0,   8, 4) == 0 { print(1) }     # elapsed 0
+    if Anim.frame(QTR,  8, 4) == 2 { print(2) }     # elapsed 2
+    if Anim.frame(HALF, 8, 4) == 0 { print(3) }     # elapsed 4 -> 4%4
+    if Anim.frame(E78,  8, 4) == 3 { print(4) }     # elapsed 7 -> 7%4
+
+    # --- Anim.once: clamped one-shot (stops on the last frame) ---
+    if Anim.once(HALF, 8, 4) == 3 { print(5) }      # elapsed 4 -> min(4,3)
+    if Anim.once(QTR,  8, 4) == 2 { print(6) }      # elapsed 2
+
+    # --- Anim.pingpong: bounce 0..3..0 ---
+    if Anim.pingpong(HALF, 8, 4) == 2 { print(7) }  # elapsed 4 -> 6-4
+    if Anim.pingpong(E18,  8, 4) == 1 { print(8) }  # elapsed 1
+
+    # --- Anim.finished: has a one-shot run past its last frame? ---
+    if Anim.finished(HALF, 8, 4)     { print(9) }   # elapsed 4 >= 4
+    if not Anim.finished(QTR, 8, 4)  { print(10) }  # elapsed 2 < 4
+
+    # --- Anim.duration + spritesheet cell geometry ---
+    if Anim.duration(8, 4) == HALF   { print(11) }  # 4/8 = 0.5s
+    if Anim.cell_x(5, 4, 16) == 16   { print(12) }  # (5%4)*16
+    if Anim.cell_y(5, 4, 16) == 16   { print(13) }  # (5/4)*16
+    if Anim.cell_x(6, 4, 16) == 32   { print(14) }  # (6%4)*16
+
+    # --- Tween.progress: clamped 0..1 over a duration ---
+    if Tween.progress(QTR, F1) == QTR       { print(15) }   # 0.25/1
+    if Tween.progress(TWO, F1) == F1        { print(16) }   # 2/1 -> clamp 1
+    if Tween.progress(F0 - F1, F1) == F0    { print(17) }   # -1 -> clamp 0
+
+    # --- Tween.done ---
+    if Tween.done(F1, F1)          { print(18) }
+    if not Tween.done(HALF, F1)    { print(19) }
+
+    # --- Tween.loop / Tween.yoyo ---
+    if Tween.loop(F125, F1) == QTR { print(20) }    # frac(1.25)
+    if Tween.yoyo(HALF, F1) == HALF { print(21) }   # up-leg
+    if Tween.yoyo(F15,  F1) == HALF { print(22) }   # down-leg 2-1.5
+
+    # --- Tween.number / Tween.round scalar interpolation ---
+    if Tween.number(F0, FOUR, HALF) == TWO { print(23) }    # lerp fixed
+    if Tween.round(0, 10, HALF) == 5       { print(24) }    # round(5.0)
+    if Tween.round(0, 10, QTR)  == 3       { print(25) }    # round(2.5) half-up
+
+    # --- Tween.tint: per-channel color blend ---
+    if Tween.tint(0, WHITE, F0)   == 0     { print(26) }
+    if Tween.tint(0, WHITE, F1)   == WHITE { print(27) }
+    if Tween.tint(0, WHITE, HALF) == MID   { print(28) }    # 0x7F7F7F
+
+    # --- Tween.point: 2D vector interpolation ---
+    let p = Tween.point(Vector.make(F0, F0), Vector.make(FOUR, fixed(8)), HALF)
+    if Vector.x(p) == TWO       { print(29) }
+    if Vector.y(p) == FOUR      { print(30) }
+
+    # --- Tween.ease: curve selection, and consistency with Ease.* ---
+    if Tween.ease(HALF, 0) == HALF        { print(31) }     # 0 = linear
+    if Tween.ease(HALF, 1) == QTR         { print(32) }     # 1 = ease-in: 0.5^2
+    if Tween.ease(HALF, 1) == Ease.in(HALF) { print(33) }   # same curve as Ease.in
+    if Tween.ease(F1, 2) == F1            { print(34) }     # 2 = ease-out at 1.0
+  }
+
+  handler Run phase Update { quit() }
+}

selfhost/backend/emit_call.ludic

@@ -23,6 +23,14 @@ function emit_ns_call(ns: pointer, meth: pointer, e: Node) -> Val {
     if is_ease_ns(meth) { return emit_ease_ns(meth, e) }
     perr(`unknown builtin Ease.{meth}`)
   }
+  if (ns == "Anim") {
+    if is_anim_ns(meth) { return emit_anim_ns(meth, e) }
+    perr(`unknown builtin Anim.{meth}`)
+  }
+  if (ns == "Tween") {
+    if is_tween_ns(meth) { return emit_tween_ns(meth, e) }
+    perr(`unknown builtin Tween.{meth}`)
+  }
   if (ns == "Collision") {
     if is_collide_ns(meth) { return emit_collide_ns(meth, e) }
     perr(`unknown builtin Collision.{meth}`)

selfhost/backend/stdlib/emit_anim.ludic

@@ -0,0 +1,174 @@
+# emit_anim.ludic — 2D animation: the Anim.* (spritesheet frame animation) and
+# Tween.* (value interpolation over a timeline) namespaces. Both are pure,
+# deterministic Q16.16 / integer math driven off the game's fixed frame clock —
+# the game stores a timer in a component and calls these each frame, exactly the
+# way Collision.* / Grid.* are used. Same inputs -> same frame and same eased
+# value on every run, so replays and lockstep netcode reproduce motion exactly.
+#
+# Conventions: a `timer` is elapsed seconds as a fixed (Q16.16); `fps` and frame
+# counts are plain ints; interpolation amounts `t` are a fixed in 0.0..1.0.
+
+# ------------------------------------------------------------------ Anim.* ----
+# spritesheet frame animation: turn an elapsed timer into the frame index to
+# draw. floor(timer * fps) is the number of whole frames elapsed; the flavour
+# (loop / once / ping-pong) decides how that maps back into 0..count-1.
+
+function is_anim_ns(meth: pointer) -> bool {
+  if (meth == "frame") or (meth == "once") or (meth == "pingpong") { return true }
+  if (meth == "finished") or (meth == "duration") { return true }
+  if (meth == "cell_x") or (meth == "cell_y") { return true }
+  return false
+}
+
+# floor(timer * fps) -> i32 code of whole frames elapsed (i64 intermediate so a
+# long-running timer can't overflow the multiply).
+function anim_elapsed(timer: pointer, fps: pointer) -> pointer {
+  let t64 = emit_bind(`sext i32 {timer} to i64`)
+  let f64 = emit_bind(`sext i32 {fps} to i64`)
+  let m = emit_bind(`mul i64 {t64}, {f64}`)         # Q16.16 frames
+  let sh = emit_bind(`ashr i64 {m}, 16`)
+  return emit_bind(`trunc i64 {sh} to i32`)
+}
+
+# max(1, v) — guard a divisor / modulus against a zero or negative count.
+function anim_atleast1(v: pointer) -> pointer {
+  let c = emit_bind(`icmp slt i32 {v}, 1`)
+  return emit_bind(`select i1 {c}, i32 1, i32 {v}`)
+}
+
+function emit_anim_ns(meth: pointer, e: Node) -> Val {
+  if (meth == "frame") {                            # looping frame: elapsed mod count
+    let timer = emit_expr(e.kids[0]);  let fps = emit_expr(e.kids[1]);  let count = emit_expr(e.kids[2])
+    let el = anim_elapsed(timer.code, fps.code)
+    let cnt = anim_atleast1(count.code)
+    return val(emit_bind(`srem i32 {el}, {cnt}`), "int")
+  }
+  if (meth == "once") {                             # one-shot: min(elapsed, count-1)
+    let timer = emit_expr(e.kids[0]);  let fps = emit_expr(e.kids[1]);  let count = emit_expr(e.kids[2])
+    let el = anim_elapsed(timer.code, fps.code)
+    let last = emit_bind(`sub i32 {count.code}, 1`)
+    let c = emit_bind(`icmp slt i32 {el}, {last}`)
+    return val(emit_bind(`select i1 {c}, i32 {el}, i32 {last}`), "int")
+  }
+  if (meth == "pingpong") {                         # bounce 0..count-1..0
+    let timer = emit_expr(e.kids[0]);  let fps = emit_expr(e.kids[1]);  let count = emit_expr(e.kids[2])
+    let el = anim_elapsed(timer.code, fps.code)
+    let two = emit_bind(`mul i32 {count.code}, 2`)
+    let p2 = emit_bind(`sub i32 {two}, 2`)          # 2*count - 2
+    let period = anim_atleast1(p2)
+    let m = emit_bind(`srem i32 {el}, {period}`)
+    let back = emit_bind(`sub i32 {period}, {m}`)
+    let c = emit_bind(`icmp slt i32 {m}, {count.code}`)
+    return val(emit_bind(`select i1 {c}, i32 {m}, i32 {back}`), "int")
+  }
+  if (meth == "finished") {                         # has a one-shot run past its last frame?
+    let timer = emit_expr(e.kids[0]);  let fps = emit_expr(e.kids[1]);  let count = emit_expr(e.kids[2])
+    let el = anim_elapsed(timer.code, fps.code)
+    let c = emit_bind(`icmp sge i32 {el}, {count.code}`)
+    return val(emit_bind(`zext i1 {c} to i32`), "bool")
+  }
+  if (meth == "duration") {                         # seconds for one cycle: count / fps -> fixed
+    let fps = emit_expr(e.kids[0]);  let count = emit_expr(e.kids[1])
+    let num = emit_bind(`shl i32 {count.code}, 16`)  # count as fixed
+    let den = anim_atleast1(fps.code)
+    return val(emit_bind(`sdiv i32 {num}, {den}`), "fixed")
+  }
+  if (meth == "cell_x") {                           # source x of a frame: (frame mod cols) * cell_w
+    let frame = emit_expr(e.kids[0]);  let cols = emit_expr(e.kids[1]);  let cw = emit_expr(e.kids[2])
+    let c1 = anim_atleast1(cols.code)
+    let col = emit_bind(`srem i32 {frame.code}, {c1}`)
+    return val(emit_bind(`mul i32 {col}, {cw.code}`), "int")
+  }
+  # cell_y — source y of a frame: (frame / cols) * cell_h
+  let frame = emit_expr(e.kids[0]);  let cols = emit_expr(e.kids[1]);  let ch = emit_expr(e.kids[2])
+  let c1 = anim_atleast1(cols.code)
+  let row = emit_bind(`sdiv i32 {frame.code}, {c1}`)
+  return val(emit_bind(`mul i32 {row}, {ch.code}`), "int")
+}
+
+# ----------------------------------------------------------------- Tween.* ----
+# value interpolation over a timeline. The timeline helpers turn (timer,
+# duration) into a normalized amount with a chosen boundary behaviour; ease()
+# shapes that amount through one of the engine's easing curves (shared with
+# Ease.*); the typed interpolators blend two endpoints by an amount.
+
+function is_tween_ns(meth: pointer) -> bool {
+  if (meth == "progress") or (meth == "loop") or (meth == "yoyo") or (meth == "done") { return true }
+  if (meth == "ease") or (meth == "number") or (meth == "round") { return true }
+  if (meth == "point") or (meth == "tint") { return true }
+  return false
+}
+
+# a positive divisor for the timeline: duration if > 0, else 1.0 (65536).
+function tween_den(dur: pointer) -> pointer {
+  let dpos = emit_bind(`icmp sgt i32 {dur}, 0`)
+  return emit_bind(`select i1 {dpos}, i32 {dur}, i32 65536`)
+}
+
+function emit_tween_ns(meth: pointer, e: Node) -> Val {
+  if (meth == "progress") {                         # clamp(timer / duration, 0, 1) -> fixed
+    let timer = emit_expr(e.kids[0]);  let dur = emit_expr(e.kids[1])
+    let dpos = emit_bind(`icmp sgt i32 {dur.code}, 0`)
+    let den = emit_bind(`select i1 {dpos}, i32 {dur.code}, i32 65536`)
+    let r = fx_div_code(timer.code, den)
+    let neg = emit_bind(`icmp slt i32 {r}, 0`)
+    let lo = emit_bind(`select i1 {neg}, i32 0, i32 {r}`)
+    let over = emit_bind(`icmp sgt i32 {lo}, 65536`)
+    let r1 = emit_bind(`select i1 {over}, i32 65536, i32 {lo}`)
+    return val(emit_bind(`select i1 {dpos}, i32 {r1}, i32 65536`), "fixed")   # dur<=0 -> done
+  }
+  if (meth == "loop") {                             # frac(timer / duration) in [0,1) -> fixed
+    let timer = emit_expr(e.kids[0]);  let dur = emit_expr(e.kids[1])
+    let den = tween_den(dur.code)
+    let r = fx_div_code(timer.code, den)
+    return val(emit_bind(`and i32 {r}, 65535`), "fixed")                      # nonneg fractional part
+  }
+  if (meth == "yoyo") {                             # triangle 0..1..0 over the duration -> fixed
+    let timer = emit_expr(e.kids[0]);  let dur = emit_expr(e.kids[1])
+    let den = tween_den(dur.code)
+    let r = fx_div_code(timer.code, den)
+    let u = emit_bind(`srem i32 {r}, 131072`)        # mod 2.0
+    let back = emit_bind(`sub i32 131072, {u}`)
+    let c = emit_bind(`icmp sle i32 {u}, 65536`)
+    return val(emit_bind(`select i1 {c}, i32 {u}, i32 {back}`), "fixed")
+  }
+  if (meth == "done") {                             # timer >= duration -> bool
+    let timer = emit_expr(e.kids[0]);  let dur = emit_expr(e.kids[1])
+    let c = emit_bind(`icmp sge i32 {timer.code}, {dur.code}`)
+    return val(emit_bind(`zext i1 {c} to i32`), "bool")
+  }
+  if (meth == "ease") {                             # ease(t, mode) -> fixed; mode is a literal 0..6
+    let t = emit_expr(e.kids[0])
+    let m = e.kids[1]
+    if (m.kind != E_INT) { perr("Tween.ease: the easing mode must be a literal int 0..6") }
+    return val(ease_eval(m.ival, t.code), "fixed")
+  }
+  if (meth == "number") {                           # lerp two fixeds by t -> fixed
+    let a = emit_expr(e.kids[0]);  let b = emit_expr(e.kids[1]);  let t = emit_expr(e.kids[2])
+    return val(fx_lerp_code(a.code, b.code, t.code), "fixed")
+  }
+  if (meth == "round") {                            # lerp two ints by t, rounded -> int
+    let a = emit_expr(e.kids[0]);  let b = emit_expr(e.kids[1]);  let t = emit_expr(e.kids[2])
+    let d = emit_bind(`sub i32 {b.code}, {a.code}`)
+    let d64 = emit_bind(`sext i32 {d} to i64`)
+    let t64 = emit_bind(`sext i32 {t.code} to i64`)
+    let p = emit_bind(`mul i64 {d64}, {t64}`)        # Q16.16
+    let p2 = emit_bind(`add i64 {p}, 32768`)         # + 0.5
+    let sh = emit_bind(`ashr i64 {p2}, 16`)
+    let dt = emit_bind(`trunc i64 {sh} to i32`)
+    return val(emit_bind(`add i32 {a.code}, {dt}`), "int")
+  }
+  if (meth == "point") {                            # lerp two Vectors by t -> Vector
+    let a = emit_expr(e.kids[0]);  let b = emit_expr(e.kids[1]);  let t = emit_expr(e.kids[2])
+    let ax = vec_x(a.code);  let ay = vec_y(a.code);  let bx = vec_x(b.code);  let by = vec_y(b.code)
+    let lx = fx_lerp_code(ax, bx, t.code)
+    let ly = fx_lerp_code(ay, by, t.code)
+    return val(vec_pack(lx, ly), "Vector")
+  }
+  # tint — blend two colors (0x00RRGGBB) by t, per channel -> int
+  let a = emit_expr(e.kids[0]);  let b = emit_expr(e.kids[1]);  let t = emit_expr(e.kids[2])
+  let r = color_lerp_ch(color_ch(a.code, "16"), color_ch(b.code, "16"), t.code)
+  let g = color_lerp_ch(color_ch(a.code, "8"),  color_ch(b.code, "8"),  t.code)
+  let bl = color_lerp_ch(color_ch(a.code, "0"),  color_ch(b.code, "0"),  t.code)
+  return val(color_pack(r, g, bl), "int")
+}

selfhost/backend/stdlib/emit_ease.ludic

@@ -1,6 +1,11 @@
 # emit_ease.ludic — the Ease.* namespace: tween curves over a normalized amount
 # t in 0.0..1.0, returning an eased fixed. All pure Q16.16, deterministic. The
 # "juice" layer that makes motion feel good (Robert Penner's easings).
+#
+# The curve math is factored into ease_eval(mode, t) so the Tween.* namespace
+# (emit_anim.ludic) can pick a curve by a small integer mode and reuse the exact
+# same formulas — one source of truth for every easing in the engine.
+#   0 linear   1 in    2 out    3 in_out    4 back    5 elastic    6 bounce
 
 function is_ease_ns(meth: pointer) -> bool {
   if (meth == "in") or (meth == "out") or (meth == "in_out") { return true }
@@ -14,50 +19,69 @@ function ease_bounce_seg(u: pointer) -> pointer {
   return fx_mul_code(uu, "495616")                  # 7.5625 * u*u
 }
 
-function emit_ease_ns(meth: pointer, e: Node) -> Val {
-  let t = emit_expr(e.kids[0])
-  if (meth == "in") {                               # ease-in quad: t*t
-    return val(fx_mul_code(t.code, t.code), "fixed")
+# ease-out bounce: four parabolic segments, selected by t (all computed, then
+# picked branch-free). Shifts/offsets are the standard 2.75-denominator set.
+function ease_bounce_code(t: pointer) -> pointer {
+  let sA = ease_bounce_seg(t)
+  let uB = emit_bind(`sub i32 {t}, 35747`);  let sB0 = ease_bounce_seg(uB);  let sB = emit_bind(`add i32 {sB0}, 49152`)
+  let uC = emit_bind(`sub i32 {t}, 53620`);  let sC0 = ease_bounce_seg(uC);  let sC = emit_bind(`add i32 {sC0}, 61440`)
+  let uD = emit_bind(`sub i32 {t}, 62557`);  let sD0 = ease_bounce_seg(uD);  let sD = emit_bind(`add i32 {sD0}, 64512`)
+  let cCD = emit_bind(`icmp slt i32 {t}, 59578`)
+  let rCD = emit_bind(`select i1 {cCD}, i32 {sC}, i32 {sD}`)
+  let cB = emit_bind(`icmp slt i32 {t}, 47663`)
+  let rB = emit_bind(`select i1 {cB}, i32 {sB}, i32 {rCD}`)
+  let cA = emit_bind(`icmp slt i32 {t}, 23831`)
+  return emit_bind(`select i1 {cA}, i32 {sA}, i32 {rB}`)
+}
+
+# evaluate easing `mode` at normalized amount `t` (a fixed code) -> fixed code.
+# The single source of truth for the engine's easing curves.
+function ease_eval(mode: int, t: pointer) -> pointer {
+  if (mode == 0) {                                  # linear: t
+    return t
   }
-  if (meth == "out") {                              # ease-out quad: t*(2 - t)
-    let inv = emit_bind(`sub i32 131072, {t.code}`)
-    return val(fx_mul_code(t.code, inv), "fixed")
+  if (mode == 1) {                                  # ease-in quad: t*t
+    return fx_mul_code(t, t)
   }
-  if (meth == "in_out") {                           # smooth ease-in-out: 3t^2 - 2t^3
-    let t2 = fx_mul_code(t.code, t.code)
-    let t3 = fx_mul_code(t2, t.code)
+  if (mode == 2) {                                  # ease-out quad: t*(2 - t)
+    let inv = emit_bind(`sub i32 131072, {t}`)
+    return fx_mul_code(t, inv)
+  }
+  if (mode == 3) {                                  # smooth ease-in-out: 3t^2 - 2t^3
+    let t2 = fx_mul_code(t, t)
+    let t3 = fx_mul_code(t2, t)
     let three = emit_bind(`mul i32 {t2}, 3`)
     let two = emit_bind(`mul i32 {t3}, 2`)
-    return val(emit_bind(`sub i32 {three}, {two}`), "fixed")
+    return emit_bind(`sub i32 {three}, {two}`)
   }
-  if (meth == "back") {                             # ease-in-back (overshoots below 0)
-    let t2 = fx_mul_code(t.code, t.code)
-    let t3 = fx_mul_code(t2, t.code)
+  if (mode == 4) {                                  # ease-in-back (overshoots below 0)
+    let t2 = fx_mul_code(t, t)
+    let t3 = fx_mul_code(t2, t)
     let a = fx_mul_code(t3, "177051")               # 2.70158 * t^3
     let b = fx_mul_code(t2, "111515")               # 1.70158 * t^2
-    return val(emit_bind(`sub i32 {a}, {b}`), "fixed")
+    return emit_bind(`sub i32 {a}, {b}`)
   }
-  if (meth == "elastic") {                          # ease-out elastic: springy overshoot that settles
+  if (mode == 5) {                                  # ease-out elastic: springy overshoot that settles
     g_uses_mathrt = true                            # 2^(-10t) * sin((10t - 0.75) * 2pi/3) + 1
-    let tt = emit_bind(`mul i32 {t.code}, 10`)       # 10t
+    let tt = emit_bind(`mul i32 {t}, 10`)            # 10t
     let ntt = emit_bind(`sub i32 0, {tt}`)           # -10t (exp2 exponent, Q16.16)
     let decay = emit_bind(`call i32 @fn_fx_exp2(i32 {ntt})`)
     let ph = emit_bind(`sub i32 {tt}, 49152`)        # 10t - 0.75
     let ang = fx_mul_code(ph, "137258")              # * (2pi/3), 2pi/3 = 137258 fixed
     let s = emit_bind(`call i32 @fn_fx_sin(i32 {ang})`)
     let osc = fx_mul_code(decay, s)
-    return val(emit_bind(`add i32 {osc}, 65536`), "fixed")
+    return emit_bind(`add i32 {osc}, 65536`)
   }
-  # ease-out bounce: four parabolic segments, selected by t (all computed, then
-  # picked branch-free). Shifts/offsets are the standard 2.75-denominator set.
-  let sA = ease_bounce_seg(t.code)
-  let uB = emit_bind(`sub i32 {t.code}, 35747`);  let sB0 = ease_bounce_seg(uB);  let sB = emit_bind(`add i32 {sB0}, 49152`)
-  let uC = emit_bind(`sub i32 {t.code}, 53620`);  let sC0 = ease_bounce_seg(uC);  let sC = emit_bind(`add i32 {sC0}, 61440`)
-  let uD = emit_bind(`sub i32 {t.code}, 62557`);  let sD0 = ease_bounce_seg(uD);  let sD = emit_bind(`add i32 {sD0}, 64512`)
-  let cCD = emit_bind(`icmp slt i32 {t.code}, 59578`)
-  let rCD = emit_bind(`select i1 {cCD}, i32 {sC}, i32 {sD}`)
-  let cB = emit_bind(`icmp slt i32 {t.code}, 47663`)
-  let rB = emit_bind(`select i1 {cB}, i32 {sB}, i32 {rCD}`)
-  let cA = emit_bind(`icmp slt i32 {t.code}, 23831`)
-  return val(emit_bind(`select i1 {cA}, i32 {sA}, i32 {rB}`), "fixed")
+  # mode == 6 — ease-out bounce
+  return ease_bounce_code(t)
+}
+
+function emit_ease_ns(meth: pointer, e: Node) -> Val {
+  let t = emit_expr(e.kids[0])
+  if (meth == "in")     { return val(ease_eval(1, t.code), "fixed") }
+  if (meth == "out")    { return val(ease_eval(2, t.code), "fixed") }
+  if (meth == "in_out") { return val(ease_eval(3, t.code), "fixed") }
+  if (meth == "back")   { return val(ease_eval(4, t.code), "fixed") }
+  if (meth == "elastic"){ return val(ease_eval(5, t.code), "fixed") }
+  return val(ease_eval(6, t.code), "fixed")         # bounce
 }

tools/x/selfhost.ludic

@@ -38,6 +38,7 @@ function selfhost_frags() -> []pointer {
   push(f, "selfhost/backend/stdlib/emit_fs.ludic")
   push(f, "selfhost/backend/stdlib/emit_list.ludic")
   push(f, "selfhost/backend/stdlib/emit_ease.ludic")
+  push(f, "selfhost/backend/stdlib/emit_anim.ludic")
   push(f, "selfhost/backend/game/emit_collide.ludic")
   push(f, "selfhost/backend/emit_mem.ludic")
   push(f, "selfhost/backend/stdlib/emit_time.ludic")

tools/x/test.ludic

@@ -104,6 +104,7 @@ function cmd_test() -> int {
   feat_case("library/noise", "", "1 2 3 4 5 6 7 8 9 10 11", "noise.ludic (Noise value/perlin/simplex/fbm/cellular determinism + range)")
   feat_case("library/regex", "", "1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18", "regex.ludic (Regex match/find/groups/classes/quantifiers/replace + linear-time safety)")
   feat_case("library/grid", "", "1 2 3 4 5 6 7 8 9 10 11 12 13", "grid.ludic (Grid line/flood/line_of_sight + A* pathfinding over the tilemap)")
+  feat_case("library/anim", "", "1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34", "anim.ludic (Anim frame/once/pingpong/cell + Tween progress/loop/yoyo/ease/number/round/point/tint)")
   feat_case("library/logging", "", "0 5 2 1", "logging.ludic (Log levels, set_level/level threshold, structured fields)")
   # Os known-folders/arch and Fs.list read the BSD utsname/dirent layout, so
   # their asserted values are macOS-specific; skip off Darwin (see is_darwin).