Place compositions in scenes and play them from behaviors (#417)

A composition can now sit in a scene like a container and play there.
Its items become real objects while they run, so behaviors, colliders
and events work on them as on any other object.

- A composition is placeable with its own symbol, transform and render
  layer. The scene view shows its picture at frame 0. A click on that
  picture selects the placement, not a single item.
- CompositionSystem spawns each item as an object under the placement
  when it begins, with the id <placement>~<item>. On start and the
  other events run. The object is destroyed when the item ends.
- The top track lies in front no matter which item started first.
  World.arrange() orders the entity list and the scene objects.
- Clips and driven videos follow the same rules as compositionAt().
  Loop, restart, stop and the game pause work.
- New nodes: Play composition (with restart), Stop composition and the
  event On composition finished. They were planned for step 4 but come
  now, because without autoplay nothing could play otherwise.
- The editor run ignores Play composition and says so in the log. It
  writes into the saved project, and the items would stay behind.
- contentId() moved to instance.ts. The animation system uses it for
  nested ids too.
- Compositions get no "Sort with parent" for now.

Tests: the gate passed with 301 files and 4643 tests, 18 more than
step 1. 13 of them are in tests/core/composition-play.test.ts. The
first gate run failed in event-order.test.ts, which counts the event
nodes and now finds 19. Five counter-checks each turned exactly their
test red. Two order tests stayed green without the fix at first and
were rebuilt. e2e 68 is new and passed with 5 own checks. Its
counter-check without arrange() failed as expected.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core/node-catalog.test.ts

@@ -5,6 +5,7 @@
 import { describe, expect, it } from 'vitest';
 import { requireNodeDef } from '../../src/core/commands/require';
 import { BUILT_IN_NODES } from '../../src/core/graph/nodes';
+import { playComposition } from '../../src/core/model/composition';
 import {
   createEmptyGraph,
   type DeclaredPort,
@@ -107,6 +108,8 @@ const nestedMover = {
 const emitTarget = fakeEntity('Sparks');
 /** Their own entity each: the animation nodes leave players on the object they are handed. */
 const dancer = fakeEntity('Dancer');
+/** A placed composition for Play and Stop composition (#417). */
+const director = fakeEntity('Director');
 /** An object whose once-reset "swing" has run out and blends back over half a second (#233). */
 const swinging = {
   ...fakeEntity('Fighter'),
@@ -1976,6 +1979,34 @@ const CASES: Record<string, Case[]> = {
       after: () => expect(dancer.animations).toEqual([]),
     },
   ],
+  PlayComposition: [
+    {
+      name: 'no object is a no-op that still continues the flow',
+      inputs: { object: null, restart: false },
+      next: 'exec',
+    },
+    {
+      name: 'starts a player on the object, and a second play leaves it running',
+      inputs: { object: director, restart: false },
+      next: 'exec',
+      after: () => {
+        expect(director.composition).toMatchObject({ time: 0, shown: false, stopped: false });
+        const run = director.composition!.run;
+        playComposition(director);
+        expect(director.composition!.run).toBe(run);
+        playComposition(director, true);
+        expect(director.composition!.run).toBeGreaterThan(run);
+      },
+    },
+  ],
+  StopComposition: [
+    {
+      name: 'marks the player stopped for the next tick to take its items away',
+      inputs: { object: director },
+      next: 'exec',
+      after: () => expect(director.composition?.stopped).toBe(true),
+    },
+  ],
   PlaySound: [
     {
       name: 'no sound is a no-op that still continues the flow',
@@ -2506,6 +2537,7 @@ const EVENT_NODES: Record<string, string> = {
   OnClick: 'nodes.test.ts',
   OnMouseWheel: 'clip-areas.test.ts',
   OnAnimationFinished: 'animation.test.ts',
+  OnCompositionFinished: 'composition-play.test.ts',
   OnStateEntered: 'state-machine.test.ts',
   OnStateExited: 'state-machine.test.ts',
 };