Multiselection and container assets: create container from selection, template instances

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/core/model/Asset.ts

@@ -45,7 +45,19 @@ export interface ColliderAsset extends AssetBase {
   type: 'collider';
 }
 
-export type Asset = SpriteAsset | CameraAsset | SceneAsset | BehaviorAsset | ColliderAsset;
+/**
+ * A container is a reusable group of objects (a "prefab"): its `objects` are a template that
+ * is instantiated below the container whenever it is placed. Template positions are local
+ * to the container's origin – the centre of the group when it was created from a selection.
+ * Template ids are private to the asset; every placed copy gets fresh ids and remembers the
+ * template object it came from (`SceneObject.templateId`).
+ */
+export interface ContainerAsset extends AssetBase {
+  type: 'container';
+  objects: SceneObject[];
+}
+
+export type Asset = SpriteAsset | CameraAsset | SceneAsset | BehaviorAsset | ColliderAsset | ContainerAsset;
 
 export function isSceneAsset(asset: Asset): asset is SceneAsset {
   return asset.type === 'scene';