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>

tests/core/holders.test.ts

@@ -9,7 +9,13 @@ import { CreateContainerCommand } from '../../src/core/commands/containerCommand
 import { SetOverrideCommand } from '../../src/core/commands/sceneObjectCommands';
 import { RemoveVariableCommand, RenameVariableCommand } from '../../src/core/commands/variableCommands';
 import type { ContainerAsset } from '../../src/core/model/Asset';
-import { findAsset, findUsages, objectHolders } from '../../src/core/model/Project';
+import {
+  findAsset,
+  findObjectHolder,
+  findUsages,
+  listContainers,
+  objectHolders,
+} from '../../src/core/model/Project';
 import { makeBehavior, makeSpriteScene, place } from '../helpers';
 
 /** A behavior with a `speed` variable, placed and overridden to 42, then wrapped in a container. */
@@ -21,7 +27,7 @@ function containerWithOverride() {
   const container = new CreateContainerCommand(f.sceneId, [objectId]);
   f.stack.execute(container);
   const template = () => (findAsset(f.project, container.assetId) as ContainerAsset).objects[0]!;
-  return { ...f, behaviorId: behavior.id, objectId, template };
+  return { ...f, behaviorId: behavior.id, containerId: container.assetId, objectId, template };
 }
 
 describe('object holders', () => {
@@ -34,6 +40,20 @@ describe('object holders', () => {
     ).toEqual(['container', 'scene']);
   });
 
+  it('resolves a scene or a container by id, but no other asset', () => {
+    const { project, sceneId, containerId, spriteId, behaviorId } = containerWithOverride();
+    expect(findObjectHolder(project, sceneId)?.type).toBe('scene');
+    expect(findObjectHolder(project, containerId)?.type).toBe('container');
+    expect(findObjectHolder(project, spriteId)).toBeUndefined();
+    expect(findObjectHolder(project, behaviorId)).toBeUndefined();
+    expect(findObjectHolder(project, 'nope')).toBeUndefined();
+  });
+
+  it('lists the container assets on their own', () => {
+    const { project, containerId } = containerWithOverride();
+    expect(listContainers(project).map((c) => c.id)).toEqual([containerId]);
+  });
+
   it('finds usages inside container templates, not just in scenes', () => {
     const { project, behaviorId, template } = containerWithOverride();
     // The placed object and the template copy the container was built from.