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-chain.md

@@ -0,0 +1,32 @@
+---
+id: tween-chain
+name: Tween.chain
+category: tween
+kind: namespace-method
+tokens: Tween.chain
+sig: Tween.chain(handle, to, dur, ease) -> handle
+tip: Append a tween segment that runs after the handle's current queue.
+order: 31
+ns: Tween
+member: chain
+---
+
+Appends a tween segment to a <a href="tween-to"><code>handle</code></a> that runs <em>after</em> its current segments finish, continuing from where the previous one ended — so the game only names the new target. Returns the same handle, so calls chain fluently. A handle holds up to eight segments; combine with <a href="tween-delay"><code>Tween.delay</code></a> for pauses. The engine advances the whole sequence one segment at a time.
+
+Parameters:
+- `handle` — the tween handle to extend
+- `to` — the target value for this segment
+- `dur` — the segment length in engine ticks
+- `ease` — 0 linear, 1 in, 2 out, 3 in-out
+
+```ludic
+program Demo {
+  property Marker { n: int = 0 }
+  model M { Marker }
+  entry {
+    var h = Tween.to(0, 100, 20, 2)     # rise
+    h = Tween.chain(h, 0, 20, 1)        # then fall back
+    quit()
+  }
+}
+```