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-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))
+  }
+}
+```