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/frontend/parse.ludic

@@ -183,6 +183,15 @@ function p_postfix() -> Node {
       # Input.* action-map / record-replay methods (#7) -> splice input.ludic.
       # Input.key stays bare (no runtime), so gate on the new methods only.
       if e.a.kind == E_ID and e.a.s == "Input" and (e.s == "bind" or e.s == "rebind" or e.s == "poll" or e.s == "down" or e.s == "pressed" or e.s == "record" or e.s == "replay") { g_uses_input = true }
+      # Anim.play/clip/on_frame/fired + Motion.to (#48): the ergonomic writes over
+      # the SpriteAnim/Motion components live in systems.ludic and use the world
+      # table, so splice it and force the reflection ABI even if the game leaves
+      # the engine auto-advance to do the ticking.
+      if e.a.kind == E_ID and e.a.s == "Anim" and (e.s == "play" or e.s == "clip" or e.s == "on_frame" or e.s == "fired") { g_uses_anim_rt = true }
+      if e.a.kind == E_ID and e.a.s == "Motion" and e.s == "to" { g_uses_anim_rt = true }
+      # Tween.to/chain/delay/value/stop/parallel (#48): the fluent stateful handles
+      # live in tween.ludic, advanced by an engine-owned system each Update tick.
+      if e.a.kind == E_ID and e.a.s == "Tween" and (e.s == "to" or e.s == "chain" or e.s == "delay" or e.s == "value" or e.s == "stop" or e.s == "parallel") { g_uses_tween_rt = true }
     }
     else { if is_op("[") { pi = pi + 1; let lo = expr()
         if is_op("..") { pi = pi + 1; let sl = node(E_SLICE); sl.a = e; sl.b = lo; sl.c = expr(); eat_op("]"); e = sl }   # s[a..b] substring
@@ -412,6 +421,8 @@ var g_uses_value: bool = false    # Value.*/Json.*/Reflect.serialize -> splice t
 var g_uses_reflect_io: bool = false  # Reflect.serialize/apply -> splice the reflection serializer
 var g_uses_esys: bool = false     # an engine-owned system component (SpriteAnim/Motion/Light2D) is declared -> splice systems.ludic + force the reflection ABI
 var g_uses_input: bool = false    # a program used Input.bind/down/poll/… (action maps + record/replay) -> splice input.ludic
+var g_uses_anim_rt: bool = false  # Anim.play/clip/on_frame/fired or Motion.to (#48) -> splice systems.ludic + force the reflection ABI
+var g_uses_tween_rt: bool = false # Tween.to/chain/delay/… (#48) -> splice tween.ludic + run esys_tween each Update
 
 function already_loaded(full: pointer) -> bool {
   var i = 0
@@ -623,6 +634,25 @@ function maybe_splice_runtime() -> void {
     do_import("runtime/native/input.ludic")
     cur_dir = saved
   }
+  # Tween.* fluent handles (#48): splice the stateful tween runtime; esys_tween is
+  # inserted into the Update phase (emit_game.ludic) to advance handles each tick.
+  # It reads the live frame clock via the standard game loop, so pull core in too.
+  if g_uses_tween_rt {
+    cur_dir = ""
+    do_import("runtime/native/core.ludic")
+    do_import("runtime/native/tween.ludic")
+    cur_dir = saved
+  }
+  # Anim.play/Motion.to sugar (#48): the writes live in systems.ludic and use the
+  # reflection ABI, so splice it and force the world table even when the game does
+  # not otherwise trip uses_engine_systems.
+  if g_uses_anim_rt {
+    g_uses_esys = true
+    cur_dir = ""
+    do_import("runtime/native/core.ludic")
+    do_import("runtime/native/systems.ludic")
+    cur_dir = saved
+  }
   if uses_engine_systems() {
     g_uses_esys = true
     cur_dir = ""