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>

src/runtime/Entity.ts

@@ -1,6 +1,7 @@
 import type { ENTITY_HANDLE } from '../core/graph/NodeContext';
 import type { AnimationPlayer } from '../core/model/animation';
 import type { AssetType } from '../core/model/AssetTypes';
+import type { CompositionPlayer } from '../core/model/composition';
 import type { FrameOutline } from '../core/model/frameOutline';
 import type { TileEdits } from '../core/model/liveTilemap';
 import type { PropertyBag } from '../core/model/Properties';
@@ -26,6 +27,8 @@ export interface Entity {
   parent: Entity | null;
   /** The animations playing on this object; kept out of `props`, which the world writes back. */
   animations?: AnimationPlayer[];
+  /** For a placed composition: what `Play composition` started (#417), out of `props` like `animations`. */
+  composition?: CompositionPlayer;
   /** How often an `Emit` node has restarted this particle source; out of `props` for the same reason. */
   emits?: number;
   /**