An animation is a timeline of rows over one asset, played on an object rather than worn as a picture (#97)

Part (a) of the animation rework: model, commands, saving, runtime and nodes.

- AnimationAsset holds a root (sprite, text, container or scene) and rows; a row keys one
  property of the root or of an object inside it, keys sit on whole frames.
- The last key decides the length (its frame counts as a frame of its own). The playhead runs
  between frames, so numbers and colors blend every tick even at 10 fps; everything else holds.
- Playback gains `once-reset`, which puts back what was there before the start. A row can be
  relative, adding to where its target stood.
- Play / Pause / Stop animation act on an object; each instance plays on its own, the later
  start wins on a shared property, and a behavior writing the same property still writes last.
- A sprite no longer wears an animation through `texture`. "Create animation from texture"
  builds a sprite plus a texture row keyed frame by frame instead.
- The Animation tab shows a placeholder until the timeline editor (part b) lands.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/graph/nodes/index.ts

@@ -1,5 +1,6 @@
 import { type NodeDef, registerNode } from '../NodeRegistry';
 import { Destroy } from './action/Destroy';
+import { PauseAnimation, PlayAnimation, StopAnimation } from './action/animationNodes';
 import { Emit } from './action/Emit';
 import { LoadScene } from './action/LoadScene';
 import { Log } from './action/Log';
@@ -204,6 +205,9 @@ export const BUILT_IN_NODES: readonly NodeDef[] = [
   Spawn,
   Destroy,
   Emit,
+  PlayAnimation,
+  PauseAnimation,
+  StopAnimation,
   PlaySound,
   StopSound,
   LoadScene,