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>

tools/lib/harness.mjs

@@ -27,6 +27,7 @@ const KEYS = {
   ArrowRight: { key: 'ArrowRight', code: 'ArrowRight', windowsVirtualKeyCode: 39 },
   Delete: { key: 'Delete', code: 'Delete', windowsVirtualKeyCode: 46 },
   a: { key: 'a', code: 'KeyA', windowsVirtualKeyCode: 65 },
+  g: { key: 'g', code: 'KeyG', windowsVirtualKeyCode: 71 },
   z: { key: 'z', code: 'KeyZ', windowsVirtualKeyCode: 90 },
 };
 
@@ -164,8 +165,15 @@ function makeTools({ cdp, projectId, baseUrl, consoleLog, check, shotPrefix }) {
     json(
       '{ selection: __gm.state.selection, mode: __gm.state.mode, save: __gm.state.saveStatus, notice: __gm.state.notice }',
     );
-  const objects = () => json('__gm.state.activeScene.objects');
-  const object = (id) => json(`__gm.state.activeScene.objects.find((o) => o.id === ${JSON.stringify(id)})`);
+  /**
+   * The scene's objects the way the editor lists them: the content of a placed container is
+   * resolved from its template and stored nowhere (see `src/core/model/instance.ts`), so it is in
+   * here but not in `state.activeScene.objects`. Use `storedObjects()` for the latter.
+   */
+  const objects = () => json('__gm.sceneObjects()');
+  const object = (id) => json(`__gm.sceneObjects().find((o) => o.id === ${JSON.stringify(id)})`);
+  /** Only what the scene really holds – a placed container counts as one object, not as its content. */
+  const storedObjects = () => json('__gm.state.activeScene.objects');
   const asset = (id) => json(`__gm.state.project.assets.find((a) => a.id === ${JSON.stringify(id)})`);
   const world = () => json('__gm.world()');
   const entity = async (objectId) => (await world()).entities.find((e) => e.objectId === objectId);
@@ -224,6 +232,7 @@ function makeTools({ cdp, projectId, baseUrl, consoleLog, check, shotPrefix }) {
     state,
     objects,
     object,
+    storedObjects,
     asset,
     world,
     entity,