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/runtime/Entity.ts

@@ -1,4 +1,5 @@
 import type { ENTITY_HANDLE } from '../core/graph/NodeContext';
+import type { AnimationPlayer } from '../core/model/animation';
 import type { AssetType } from '../core/model/AssetTypes';
 import type { PropertyBag } from '../core/model/Properties';
 
@@ -21,14 +22,8 @@ export interface Entity {
   props: PropertyBag;
   /** Parent entity (mirrors `SceneObject.parentId`); children move and die with it. */
   parent: Entity | null;
-  /** The animation a sprite wears and how far it is; kept out of `props`, which the world writes back. */
-  animation?: AnimationClock;
+  /** The animations playing on this object; kept out of `props`, which the world writes back. */
+  animations?: AnimationPlayer[];
   /** How often an `Emit` node has restarted this particle source; out of `props` for the same reason. */
   emits?: number;
 }
-
-export interface AnimationClock {
-  id: string;
-  time: number;
-  finished: boolean;
-}