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-clip.md

@@ -0,0 +1,35 @@
+---
+id: anim-clip
+name: Anim.clip
+category: anim
+kind: namespace-method
+tokens: Anim.clip
+sig: Anim.clip(name, frames, fps, mode)
+tip: Register a named spritesheet clip so Anim.play can play it by name.
+order: 31
+ns: Anim
+member: clip
+---
+
+Registers (or updates) a spritesheet clip under a <code>name</code>, so gameplay refers to a motion by name — <code>Anim.play(entity, "run")</code> — instead of repeating its frame count and rate everywhere. The clip stores <code>frames</code> (cell count), <code>fps</code> (rate) and <code>mode</code> (0 loop, 1 once, 2 ping-pong). Names compare by identity (a string literal interns to one pointer), and the registry holds up to 32 clips. Registering the same name again overwrites it.
+
+Parameters:
+- `name` — the clip name (a string)
+- `frames` — number of cells in the clip
+- `fps` — playback rate in frames per second
+- `mode` — 0 loop, 1 once (clamp on last), 2 ping-pong
+
+```ludic
+program Demo {
+  property SpriteAnim { ticks: int = 0, fps: int = 0, frames: int = 0, mode: int = 0, frame: int = 0 }
+  model Hero { SpriteAnim }
+  entry {
+    Anim.clip("idle", 2, 4, 0)          # a slow 2-frame idle loop
+    Anim.clip("run",  6, 12, 0)         # a 6-frame run
+    Anim.clip("hit",  3, 15, 1)         # a one-shot hit reaction
+    spawn Hero { SpriteAnim { } }
+    Anim.play(World.query_next(World.prop_id("SpriteAnim"), 0), "run")
+    quit()
+  }
+}
+```