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>

src/core/model/Asset.ts

@@ -41,9 +41,9 @@ export interface ColliderAsset extends AssetBase {
 }
 
 /**
- * `objects` is a template instantiated below the container whenever it is placed. Template
- * positions are local to the container's origin, and every placed copy gets fresh ids and
- * remembers the template object it came from (`SceneObject.templateId`).
+ * `objects` is a template, not content: a placed container shows it without copying it, so an
+ * edit here reaches every instance (see `model/instance.ts`). Positions are local to the
+ * container's origin.
  */
 export interface ContainerAsset extends AssetBase {
   type: 'container';