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/tween-parallel.md

@@ -0,0 +1,30 @@
+---
+id: tween-parallel
+name: Tween.parallel
+category: tween
+kind: namespace-method
+tokens: Tween.parallel
+sig: Tween.parallel(a, b) -> bool
+tip: Are two tween handles both finished? — a parallel completion query.
+order: 35
+ns: Tween
+member: parallel
+---
+
+Returns whether two <a href="tween-to"><code>handles</code></a> are <em>both</em> finished. Independent handles are advanced together every frame, so running several at once already runs them in parallel; this is the "wait for all" query over a pair — start a fade and a slide together, then act when both complete. For longer sequences on a single value, chain segments with <a href="tween-chain"><code>Tween.chain</code></a> instead.
+
+Parameters:
+- `a`, `b` — the two tween handles to test
+
+```ludic
+program Demo {
+  property Marker { n: int = 0 }
+  model M { Marker }
+  entry {
+    let fade  = Tween.to(255, 0, 20, 0)
+    let slide = Tween.to(0, 80, 30, 2)
+    if Tween.parallel(fade, slide) { print(1) }   # both done?
+    quit()
+  }
+}
+```