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/editor/debugHook.ts

@@ -1,4 +1,6 @@
 import { type Application, type Container, Rectangle, Text } from 'pixi.js';
+import { resolveObjects } from '../core/model/instance';
+import { findObjectHolder } from '../core/model/Project';
 import { Button, ui, Widget } from '../ui';
 import type { CentreTab, Editor } from './Editor';
 import type { EditorState } from './state';
@@ -103,8 +105,18 @@ export function installDebugHook(app: Application, state: EditorState, editor: E
     const p = popup.getGlobalPosition();
     return { x: p.x, y: p.y, w: popup.w, h: popup.h };
   };
+  /**
+   * A holder's objects the way the editor sees them: the content of placed containers is resolved
+   * from the template, so it is nowhere in `holder.objects` (see `model/instance.ts`).
+   */
+  const objectsOf = (holderId: string) => {
+    const holder = findObjectHolder(state.project, holderId);
+    return holder ? resolveObjects(state.project, holder) : [];
+  };
   (window as unknown as { __gm: unknown }).__gm = {
     state,
+    objectsOf,
+    sceneObjects: () => (state.activeScene ? objectsOf(state.activeScene.id) : []),
     locate,
     overlayTexts,
     overlayRect,