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/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)
+  }
+}
+```