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/_section.md

@@ -4,4 +4,4 @@ title: Anim
 order: 28
 ---
 
-Spritesheet frame animation off the fixed frame clock. Store an elapsed <code>timer</code> (seconds, a <code>fixed</code>) on a component and each frame ask <a href="anim-frame"><code>Anim.frame</code></a> / <a href="anim-once"><code>Anim.once</code></a> / <a href="anim-pingpong"><code>Anim.pingpong</code></a> which cell to draw; <a href="anim-cell_x"><code>Anim.cell_x</code></a>/<a href="anim-cell_y"><code>Anim.cell_y</code></a> turn a frame index into a source rectangle on the sheet. Everything is integer/fixed and deterministic — the same timer reproduces the same frame every run, so replays and lockstep netcode match exactly.
+Spritesheet frame animation off the fixed frame clock. Store an elapsed <code>timer</code> (seconds, a <code>fixed</code>) on a component and each frame ask <a href="anim-frame"><code>Anim.frame</code></a> / <a href="anim-once"><code>Anim.once</code></a> / <a href="anim-pingpong"><code>Anim.pingpong</code></a> which cell to draw; <a href="anim-cell_x"><code>Anim.cell_x</code></a>/<a href="anim-cell_y"><code>Anim.cell_y</code></a> turn a frame index into a source rectangle on the sheet. Everything is integer/fixed and deterministic — the same timer reproduces the same frame every run, so replays and lockstep netcode match exactly. For the ECS engine-owned <code>SpriteAnim</code> component there is also an ergonomic layer: register named clips with <a href="anim-clip"><code>Anim.clip</code></a> and play them by name with <a href="anim-play"><code>Anim.play</code></a>, and arm frame events with <a href="anim-on_frame"><code>Anim.on_frame</code></a> / <a href="anim-fired"><code>Anim.fired</code></a>.

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()
+  }
+}
+```

docs/language/anim/anim-fired.md

@@ -0,0 +1,28 @@
+---
+id: anim-fired
+name: Anim.fired
+category: anim
+kind: namespace-method
+tokens: Anim.fired
+sig: Anim.fired(entity) -> bool
+tip: Did the entity's clip land on its armed frame event this tick?
+order: 33
+ns: Anim
+member: fired
+---
+
+Returns whether an entity's <code>SpriteAnim</code> clip landed on the frame armed by <a href="anim-on_frame"><code>Anim.on_frame</code></a> this tick — the read side of a frame event. It is true only on the single tick the clip first reaches that cell, so a game can poll it each frame and turn it into a gameplay action (emit an event, spawn a hitbox, play a footstep). Returns <code>false</code> if the entity has no <code>SpriteAnim</code> or no <code>event_fired</code> field.
+
+Parameters:
+- `entity` — the entity carrying `SpriteAnim`
+
+```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 }
+  handler Step phase Update {
+    let e = World.query_next(World.prop_id("SpriteAnim"), 0)
+    if Anim.fired(e) { print(42) }      # react: footstep, hitbox, event…
+  }
+}
+```

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()
+  }
+}
+```

docs/language/anim/anim-play.md

@@ -0,0 +1,34 @@
+---
+id: anim-play
+name: Anim.play
+category: anim
+kind: namespace-method
+tokens: Anim.play
+sig: Anim.play(entity, clip) | Anim.play(entity, fps, frames, mode)
+tip: Start (or restart) a spritesheet clip on an entity in one call.
+order: 30
+ns: Anim
+member: play
+---
+
+Starts a spritesheet clip on an entity's <code>SpriteAnim</code> component and rewinds it to its first cell, so gameplay swaps or replays an animation with one call instead of setting five fields by hand. Two forms: <code>Anim.play(entity, "run")</code> plays a <a href="anim-clip"><code>named clip</code></a> registered with <code>Anim.clip</code>; <code>Anim.play(entity, fps, frames, mode)</code> sets the clip directly (<code>mode</code> 0 loop, 1 once, 2 ping-pong). A no-op if the entity has no <code>SpriteAnim</code>. The engine advances the clip from there each tick.
+
+Parameters:
+- `entity` — the entity carrying `SpriteAnim`
+- `clip` — a registered clip name (2-argument form), **or**
+- `fps`, `frames`, `mode` — the clip rate, cell count and play mode (4-argument form)
+
+```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("run", 6, 12, 0)
+    spawn Hero { SpriteAnim { } }
+    let e = World.query_next(World.prop_id("SpriteAnim"), 0)
+    Anim.play(e, "run")                 # by name
+    Anim.play(e, 8, 4, 1)               # or directly: 4 frames @ 8fps, once
+    quit()
+  }
+}
+```