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/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)
+  }
+}
+```