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.