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/NodeContext.ts

@@ -1,3 +1,4 @@
+import type { AnimationPlayer } from '../model/animation';
 import type { Mat } from '../model/matrix';
 import type { Position, PropertyBag } from '../model/Properties';
 import { type ChainTransform, composeAlongChain } from '../model/walk';
@@ -28,9 +29,11 @@ export interface EntityHandle {
   parent: EntityHandle | null;
   /**
    * Counts the Emit pulses a particle source has been given. Kept off `props` for the reason the
-   * animation clock is: what the world writes back to the model must not grow an override per frame.
+   * animation players are: what the world writes back to the model must not grow an override per frame.
    */
   emits?: number;
+  /** What `Play animation` started on this object (see `model/animation.ts`). */
+  animations?: AnimationPlayer[];
 }
 
 /** Position, rotation, scale and mirror of every ancestor applied, nearest last. */

src/core/graph/nodes/action/animationNodes.ts

@@ -0,0 +1,64 @@
+import { pauseAnimation, playAnimation, stopAnimation } from '../../../model/animation';
+import { textOf } from '../../../model/Properties';
+import type { PropertyDef } from '../../../properties/PropertySchema';
+import type { NodeDef } from '../../NodeRegistry';
+import { asEntity } from '../../values';
+import { exec, obj } from '../shared';
+
+const animationParam = (): PropertyDef => ({
+  key: 'animation',
+  label: 'Animation',
+  kind: 'asset',
+  default: '',
+  assetTypes: ['animation'],
+});
+
+export const PlayAnimation: NodeDef = {
+  type: 'PlayAnimation',
+  label: 'Play animation',
+  category: 'action',
+  description:
+    'Plays an animation on an object: its rows reach the object and what is placed inside it, so every ' +
+    'instance plays on its own. A paused one runs on from where it stands; one already running is left alone.',
+  inputs: [exec(), obj('object')],
+  outputs: [exec()],
+  params: [animationParam()],
+  run: (_ctx, inputs, params) => {
+    const target = asEntity(inputs.object);
+    const animation = textOf(params.animation ?? '');
+    if (target && animation) playAnimation(target, animation);
+    return { next: 'exec' };
+  },
+};
+
+export const StopAnimation: NodeDef = {
+  type: 'StopAnimation',
+  label: 'Stop animation',
+  category: 'action',
+  description:
+    'Ends an animation on an object where it stands – the values it set stay. Leave Animation at None to end every one.',
+  inputs: [exec(), obj('object')],
+  outputs: [exec()],
+  params: [animationParam()],
+  run: (_ctx, inputs, params) => {
+    const target = asEntity(inputs.object);
+    if (target) stopAnimation(target, textOf(params.animation ?? ''));
+    return { next: 'exec' };
+  },
+};
+
+export const PauseAnimation: NodeDef = {
+  type: 'PauseAnimation',
+  label: 'Pause animation',
+  category: 'action',
+  description:
+    'Holds an animation on an object at its frame until Play animation lets it run on. Leave Animation at None to hold every one.',
+  inputs: [exec(), obj('object')],
+  outputs: [exec()],
+  params: [animationParam()],
+  run: (_ctx, inputs, params) => {
+    const target = asEntity(inputs.object);
+    if (target) pauseAnimation(target, textOf(params.animation ?? ''));
+    return { next: 'exec' };
+  },
+};

src/core/graph/nodes/data/literals.ts

@@ -28,7 +28,7 @@ export const StringLiteral = literal('String', 'text', 'A constant text.');
 export const TextureLiteral = literal(
   'Texture',
   'texture',
-  'A constant picture: a texture, one of its frames or an animation, as a sprite wears it.',
+  'A constant picture: a texture or one of its frames, as a sprite wears it.',
 );
 export const StateMachineLiteral = literal(
   'StateMachine',

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,