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/anim/_section.md

@@ -4,4 +4,4 @@ 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.
+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. For the ECS engine-owned <code>SpriteAnim</code> component there is also an ergonomic layer: register named clips with <a href="anim-clip"><code>Anim.clip</code></a> and play them by name with <a href="anim-play"><code>Anim.play</code></a>, and arm frame events with <a href="anim-on_frame"><code>Anim.on_frame</code></a> / <a href="anim-fired"><code>Anim.fired</code></a>.