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/editor/scene-visibility.test.ts

@@ -27,6 +27,7 @@ describe('scene visibility', () => {
       'camera',
       'clip',
       'collider',
+      'composition',
       'container',
       'joint',
       'particles',