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/anim-on_frame.md

@@ -0,0 +1,33 @@
+---
+id: anim-on_frame
+name: Anim.on_frame
+category: anim
+kind: namespace-method
+tokens: Anim.on_frame
+sig: Anim.on_frame(entity, frame)
+tip: Arm a frame event — the engine flags the tick a clip lands on this frame.
+order: 32
+ns: Anim
+member: on_frame
+---
+
+Arms a <strong>frame event</strong> on an entity's <code>SpriteAnim</code>: the engine sets the component's <code>event_fired</code> flag on the tick the clip <em>first lands</em> on <code>frame</code> — a footstep on the contact cell, a hitbox going live mid-swing. Gameplay reads the flag with <a href="anim-fired"><code>Anim.fired</code></a> in its own handler and reacts (emit its own event, spawn, play a sound). The engine detects the boundary; the game owns the reaction, so it stays inside the deterministic, no-runtime-dispatch event model. Requires the component to carry <code>event_frame</code> and <code>event_fired</code> fields.
+
+Parameters:
+- `entity` — the entity carrying `SpriteAnim`
+- `frame` — the cell index that should fire the event
+
+```ludic
+program Demo {
+  property SpriteAnim { ticks: int = 0, fps: int = 0, frames: int = 0, mode: int = 0, frame: int = 0, event_frame: int = 0, event_fired: int = 0 }
+  model Hero { SpriteAnim }
+  entry {
+    spawn Hero { SpriteAnim { fps: 12, frames: 6, mode: 0 } }
+    let e = World.query_next(World.prop_id("SpriteAnim"), 0)
+    Anim.on_frame(e, 3)                 # fire when the run clip hits its contact frame
+    tick_fixed()
+    if Anim.fired(e) { print(1) }
+    quit()
+  }
+}
+```