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/Project.ts

@@ -1,9 +1,13 @@
 import { schemaForAsset } from '../properties/PropertySchema';
 import { type Asset, type ContainerAsset, isSceneAsset, type SceneAsset } from './Asset';
+import { overrideSites } from './instance';
 import type { SceneObject } from './SceneObject';
 
-/** Reserved for format migrations – the loader does not branch on it yet. */
-export const PROJECT_FORMAT_VERSION = 1;
+/**
+ * 2 since a placed container stopped copying its template into the scene. Old files are not
+ * migrated: the loader simply reads no container content from them any more.
+ */
+export const PROJECT_FORMAT_VERSION = 2;
 
 export interface Project {
   formatVersion: number;
@@ -82,7 +86,8 @@ export function findUsages(project: Project, assetId: string): SceneObjectRef[]
 
 export type PropertyUsage =
   | { on: 'asset'; asset: Asset; key: string }
-  | { on: 'object'; holder: ObjectHolder; object: SceneObject; key: string };
+  /** `path` is empty for a placed object's own override, set for one a container instance keeps. */
+  | { on: 'object'; holder: ObjectHolder; object: SceneObject; path: string; key: string };
 
 /**
  * The other half of "where is this asset used": a texture is never placed, it is worn through a
@@ -102,11 +107,10 @@ export function findPropertyUsages(project: Project, ids: string | readonly stri
     refKeys.set(asset.id, keys);
     for (const key of keys) if (matches(asset.props[key])) result.push({ on: 'asset', asset, key });
   }
-  for (const holder of objectHolders(project)) {
-    for (const object of holder.objects) {
-      for (const key of refKeys.get(object.assetId) ?? [])
-        if (matches(object.overrides[key])) result.push({ on: 'object', holder, object, key });
-    }
+  for (const site of overrideSites(project, () => true)) {
+    for (const key of refKeys.get(site.assetId) ?? [])
+      if (matches(site.bag[key]))
+        result.push({ on: 'object', holder: site.holder, object: site.object, path: site.path, key });
   }
   return result;
 }