Resolve container content from the template instead of copying it

A scene now stores only the container object; its content is resolved from
the template on every read, with deviations kept as sparse instanceEdits on
the container object. This drops the propagation bookkeeping entirely --
templateSync.ts is gone and the three structural gaps (add, remove, reparent)
cannot diverge any more.

Values stay editable on an instance, structure does not; Unpack writes the
content into the scene, keeping ids. Play mode bakes containers into the
throwaway copy before entities are built. No migration: formatVersion is 2
and the seeders write the new format.

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

tests/core/serialize.test.ts

@@ -56,7 +56,7 @@ describe('serialize', () => {
       editorOffset: { x: 0, y: 0 },
       overrides: { color: 0x00ff00, width: 1 },
       parentId: '',
-      templateId: '',
+      instanceEdits: {},
     });
     expect(project.assets[1]!.name).toBe('sprite');
     expect(project.assets[1]!.props).toEqual({