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>

src/core/model/Project.ts

@@ -37,8 +37,26 @@ export function listScenes(project: Project): SceneAsset[] {
  */
 export type ObjectHolder = SceneAsset | ContainerAsset;
 
+export function isObjectHolder(asset: Asset): asset is ObjectHolder {
+  return asset.type === 'scene' || asset.type === 'container';
+}
+
 export function objectHolders(project: Project): ObjectHolder[] {
-  return project.assets.filter((a): a is ObjectHolder => a.type === 'scene' || a.type === 'container');
+  return project.assets.filter(isObjectHolder);
+}
+
+/**
+ * The scene or container asset an object belongs to. Editing a container's template is the same
+ * work as editing a scene, so the object commands resolve their target through this rather than
+ * through `findScene`.
+ */
+export function findObjectHolder(project: Project, holderId: string): ObjectHolder | undefined {
+  const asset = findAsset(project, holderId);
+  return asset && isObjectHolder(asset) ? asset : undefined;
+}
+
+export function listContainers(project: Project): ContainerAsset[] {
+  return project.assets.filter((a): a is ContainerAsset => a.type === 'container');
 }
 
 export interface SceneObjectRef {