An animation is a value a graph can pick, and playing a running one makes it win again (#97)

Play, Pause and Stop animation take the animation on a port instead of a
fixed setting, fed by the new Animation literal or a list item tagged
`$animation`. Playing an animation already running keeps its frame but
counts it as started last, so turning back to a walk that never stopped
shows that walk. The actor demo walks through one Play animation over
`ItemAt(walk, facing)` again and stops every animation to stand.

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

src/core/graph/nodes/index.ts

@@ -33,6 +33,7 @@ import { Join } from './data/Join';
 import { KeyIsDown } from './data/KeyIsDown';
 import { Length } from './data/Length';
 import {
+  AnimationLiteral,
   BooleanLiteral,
   ListLiteral,
   NumberLiteral,
@@ -156,6 +157,7 @@ export const BUILT_IN_NODES: readonly NodeDef[] = [
   StateMachineLiteral,
   SoundLiteral,
   SceneLiteral,
+  AnimationLiteral,
   Add,
   Subtract,
   Multiply,