feat(anim): animation ergonomics — named clips, Anim.play/Motion.to, frame events, fluent Tween handles (#48)

The ergonomic layer over the engine-owned SpriteAnim/Motion systems (#43):

- Named clips: Anim.clip("run", frames, fps, mode) registers a clip by name and
  Anim.play(entity, "run") plays it; Anim.play(entity, fps, frames, mode) sets
  the clip directly. A name-keyed registry in systems.ludic.
- Frame events: Anim.on_frame(entity, frame) arms optional SpriteAnim
  event_frame/event_fired fields; the engine flags the tick the clip first lands
  on that frame, and Anim.fired(entity) reads it — the game reacts, so it stays
  inside the no-runtime-dispatch event model.
- Motion.to(entity, from, to, dur, ease) starts a value tween over the Motion
  component in one call (reflection-ABI writes, resetting the timer).
- Fluent Tween handles (runtime/native/tween.ludic): Tween.to / Tween.chain /
  Tween.delay build a sequenced, disposable handle advanced by a new engine-owned
  system (esys_tween, run each Update tick); Tween.value / Tween.done /
  Tween.parallel / Tween.stop read and control it. The 1-arg Tween.done(handle)
  is disambiguated from the 2-arg pure Tween.done(timer, dur).

Splicing: g_uses_anim_rt pulls in systems.ludic; g_uses_tween_rt pulls in
tween.ludic and inserts esys_tween into the Update phase. All integer and
deterministic, so animation and motion reproduce exactly under replay/lockstep.

Worked example + regression: examples/library/anim_sugar.ludic
(4 8 2 1 0 100 100 0 0 1 20 20 30 0 1). Twelve new docs pages (Anim, the new
Motion namespace, Tween handles). Full suite 77 passed, self-host C-free fixpoint
intact, no golden drift.

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

docs/language/tween/_section.md

@@ -4,4 +4,4 @@ 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.
+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. Beyond these pure interpolators there are <strong>fluent handles</strong> the engine advances for you: <a href="tween-to"><code>Tween.to</code></a> starts one and returns a handle, <a href="tween-chain"><code>Tween.chain</code></a> and <a href="tween-delay"><code>Tween.delay</code></a> sequence more segments, and <a href="tween-value"><code>Tween.value</code></a> / <a href="tween-done"><code>Tween.done</code></a> / <a href="tween-parallel"><code>Tween.parallel</code></a> / <a href="tween-stop"><code>Tween.stop</code></a> read and control them.

docs/language/tween/tween-chain.md

@@ -0,0 +1,32 @@
+---
+id: tween-chain
+name: Tween.chain
+category: tween
+kind: namespace-method
+tokens: Tween.chain
+sig: Tween.chain(handle, to, dur, ease) -> handle
+tip: Append a tween segment that runs after the handle's current queue.
+order: 31
+ns: Tween
+member: chain
+---
+
+Appends a tween segment to a <a href="tween-to"><code>handle</code></a> that runs <em>after</em> its current segments finish, continuing from where the previous one ended — so the game only names the new target. Returns the same handle, so calls chain fluently. A handle holds up to eight segments; combine with <a href="tween-delay"><code>Tween.delay</code></a> for pauses. The engine advances the whole sequence one segment at a time.
+
+Parameters:
+- `handle` — the tween handle to extend
+- `to` — the target value for this segment
+- `dur` — the segment length in engine ticks
+- `ease` — 0 linear, 1 in, 2 out, 3 in-out
+
+```ludic
+program Demo {
+  property Marker { n: int = 0 }
+  model M { Marker }
+  entry {
+    var h = Tween.to(0, 100, 20, 2)     # rise
+    h = Tween.chain(h, 0, 20, 1)        # then fall back
+    quit()
+  }
+}
+```

docs/language/tween/tween-delay.md

@@ -0,0 +1,31 @@
+---
+id: tween-delay
+name: Tween.delay
+category: tween
+kind: namespace-method
+tokens: Tween.delay
+sig: Tween.delay(handle, ticks) -> handle
+tip: Append a pause to a tween handle's sequence.
+order: 32
+ns: Tween
+member: delay
+---
+
+Appends a <strong>pause</strong> of <code>ticks</code> ticks to a <a href="tween-to"><code>handle</code></a>'s sequence: the value holds where the previous segment left it, then the next <a href="tween-chain"><code>chained</code></a> segment begins. Returns the handle for fluent chaining. Use it to stagger a multi-step motion — rise, hold, fall — or to offset one handle from another running in parallel.
+
+Parameters:
+- `handle` — the tween handle to extend
+- `ticks` — how many engine ticks to hold
+
+```ludic
+program Demo {
+  property Marker { n: int = 0 }
+  model M { Marker }
+  entry {
+    var h = Tween.to(0, 50, 15, 0)      # move
+    h = Tween.delay(h, 10)              # wait
+    h = Tween.chain(h, 100, 15, 0)      # move again
+    quit()
+  }
+}
+```

docs/language/tween/tween-parallel.md

@@ -0,0 +1,30 @@
+---
+id: tween-parallel
+name: Tween.parallel
+category: tween
+kind: namespace-method
+tokens: Tween.parallel
+sig: Tween.parallel(a, b) -> bool
+tip: Are two tween handles both finished? — a parallel completion query.
+order: 35
+ns: Tween
+member: parallel
+---
+
+Returns whether two <a href="tween-to"><code>handles</code></a> are <em>both</em> finished. Independent handles are advanced together every frame, so running several at once already runs them in parallel; this is the "wait for all" query over a pair — start a fade and a slide together, then act when both complete. For longer sequences on a single value, chain segments with <a href="tween-chain"><code>Tween.chain</code></a> instead.
+
+Parameters:
+- `a`, `b` — the two tween handles to test
+
+```ludic
+program Demo {
+  property Marker { n: int = 0 }
+  model M { Marker }
+  entry {
+    let fade  = Tween.to(255, 0, 20, 0)
+    let slide = Tween.to(0, 80, 30, 2)
+    if Tween.parallel(fade, slide) { print(1) }   # both done?
+    quit()
+  }
+}
+```

docs/language/tween/tween-stop.md

@@ -0,0 +1,29 @@
+---
+id: tween-stop
+name: Tween.stop
+category: tween
+kind: namespace-method
+tokens: Tween.stop
+sig: Tween.stop(handle)
+tip: Stop and dispose a tween handle, freezing its value.
+order: 34
+ns: Tween
+member: stop
+---
+
+Stops a <a href="tween-to"><code>tween handle</code></a> immediately and frees its slot back to the pool. Its <a href="tween-value"><code>value</code></a> is frozen where it was, and <a href="tween-done"><code>Tween.done</code></a> then reports it finished. Use it to cancel a motion early — an interrupted animation, an entity that despawned mid-tween — so long-lived games recycle handles instead of exhausting the pool.
+
+Parameters:
+- `handle` — the tween handle to stop
+
+```ludic
+program Demo {
+  property Marker { n: int = 0 }
+  model M { Marker }
+  entry {
+    let h = Tween.to(0, 100, 60, 0)
+    Tween.stop(h)                       # cancel it now
+    quit()
+  }
+}
+```

docs/language/tween/tween-to.md

@@ -0,0 +1,34 @@
+---
+id: tween-to
+name: Tween.to
+category: tween
+kind: namespace-method
+tokens: Tween.to
+sig: Tween.to(from, to, dur, ease) -> handle
+tip: Start a fluent, engine-advanced tween and return a handle.
+order: 30
+ns: Tween
+member: to
+---
+
+Starts a <strong>stateful tween handle</strong> — from <code>from</code> to <code>to</code> over <code>dur</code> ticks with easing <code>ease</code> — and returns an integer handle. Unlike the pure <a href="tween-progress"><code>Tween.progress</code></a> interpolators (which the game drives from its own timer), a handle is <em>advanced by the engine</em> one tick per frame: fire it once, then read <a href="tween-value"><code>Tween.value</code></a> each frame and <a href="tween-done"><code>Tween.done</code></a> to know when it finishes. <a href="tween-chain"><code>Tween.chain</code></a> and <a href="tween-delay"><code>Tween.delay</code></a> append segments for a sequence. Integer and deterministic, so a sequence plays identically under replay. The handle pool is fixed-size and reuses finished slots.
+
+Parameters:
+- `from`, `to` — the start and end values (integer game units)
+- `dur` — the segment length in engine ticks
+- `ease` — 0 linear, 1 in, 2 out, 3 in-out
+
+```ludic
+program Demo {
+  property SpriteAnim { ticks: int = 0, fps: int = 0, frames: int = 0, mode: int = 0, frame: int = 0 }
+  property Pop { h: int = 0 }
+  model Coin { SpriteAnim, Pop }
+  entry {
+    spawn Coin { Pop { h: 0 } }
+    let e = World.query_next(World.prop_id("Pop"), 0)
+    let f = World.field_id(World.prop_id("Pop"), "h")
+    World.set(e, World.prop_id("Pop"), f, Tween.to(0, 40, 20, 2))   # pop upward
+    quit()
+  }
+}
+```

docs/language/tween/tween-value.md

@@ -0,0 +1,32 @@
+---
+id: tween-value
+name: Tween.value
+category: tween
+kind: namespace-method
+tokens: Tween.value
+sig: Tween.value(handle) -> int
+tip: The current value of a fluent tween handle this frame.
+order: 33
+ns: Tween
+member: value
+---
+
+Returns the current interpolated value of a <a href="tween-to"><code>tween handle</code></a> this frame — the number the engine advanced it to. Read it each frame to drive a position, an alpha, a scale. Once every segment has finished the value rests on the final target; <a href="tween-done"><code>Tween.done</code></a> reports when that has happened. Returns <code>0</code> for an invalid handle.
+
+Parameters:
+- `handle` — the tween handle to read
+
+```ludic
+program Demo {
+  property Pos { x: int = 0 }
+  property Tw { h: int = 0 }
+  model Mover { Pos, Tw }
+  handler Move phase Update {
+    let p = World.prop_id("Pos")
+    let t = World.prop_id("Tw")
+    let e = World.query_next(t, 0)
+    let h = World.get(e, t, World.field_id(t, "h"))
+    World.set(e, p, World.field_id(p, "x"), Tween.value(h))
+  }
+}
+```