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>

selfhost/backend/stdlib/emit_anim.ludic

@@ -132,8 +132,14 @@ function emit_tween_ns(meth: pointer, e: Node) -> Val {
     let c = emit_bind(`icmp sle i32 {u}, 65536`)
     return val(emit_bind(`select i1 {c}, i32 {u}, i32 {back}`), "fixed")
   }
-  if (meth == "done") {                             # timer >= duration -> bool
-    let timer = emit_expr(e.kids[0]);  let dur = emit_expr(e.kids[1])
+  if (meth == "done") {
+    # 1-arg Tween.done(handle) -> the fluent stateful handle's completion (#48),
+    # a call into tween.ludic; the 2-arg Tween.done(timer, dur) is the pure form.
+    if (len(e.kids) == 1) {
+      let h = emit_expr(e.kids[0])
+      return val(emit_bind(`call i32 @fn_tween_done(i32 {h.code})`), "bool")
+    }
+    let timer = emit_expr(e.kids[0]);  let dur = emit_expr(e.kids[1])   # timer >= duration -> bool
     let c = emit_bind(`icmp sge i32 {timer.code}, {dur.code}`)
     return val(emit_bind(`zext i1 {c} to i32`), "bool")
   }