Add a Container tab and propagate template edits

The Container tab reuses SceneViewport against any object holder, so a
container template is edited exactly like a scene. Right-click no longer
changes the selection, which previously made nesting impossible: opening
the menu on a container loaded it, and "Place in container" then targeted
itself.

Template edits (move, rename, override) now reach already-placed instances
per slot, through a shared templateSync used by both the apply path and the
plain object commands. Copies the user has touched keep their own value.

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

tools/e2e/03-play.mjs

@@ -80,7 +80,7 @@ export async function play(t) {
     Boolean(await evaluate(`__gm.locate('INSPECTOR')`)) &&
       (await evaluate('__gm.hierarchyExpandAll(); __gm.hierarchyOrder().length')) === 7,
   );
-  await select({ kind: 'object', sceneId: 'scene1', objectId: 'o2' });
+  await select({ kind: 'object', holderId: 'scene1', objectId: 'o2' });
   await mutate(`__gm.state.activeScene.objects.find(o => o.id === 'o2').position.x = 400`);
   let liveO2 = await entity('o2');
   check(
@@ -158,7 +158,7 @@ export async function play(t) {
   await mutate(`__gm.state.activeScene.objects.find(o => o.id === 'o2').position.x = ${xBeforePlay}`);
 
   // 7c. Without a camera nothing is rendered: delete the camera object, play, then undo.
-  await select({ kind: 'object', sceneId: 'scene1', objectId: 'o3' });
+  await select({ kind: 'object', holderId: 'scene1', objectId: 'o3' });
   await press('Delete');
   await sleep(200);
   check('camera object deleted', (await objects()).length === 6);