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

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

src/core/model/SceneObject.ts

@@ -22,6 +22,12 @@ export interface SceneObject {
    * acts on.
    */
   parentId: string;
+  /**
+   * For objects that belong to a placed container: the id of the template object (inside
+   * the container asset) this one was instantiated from. Empty for everything else. This is
+   * the link that lets template changes reach the instances later on.
+   */
+  templateId: string;
 }
 
 /** Creates a scene object; `z` defaults to 0 when the caller only has a 2D point. */
@@ -33,5 +39,6 @@ export function createSceneObject(id: string, assetId: string, position: Vec2 |
     position: { x: position.x, y: position.y, z: 'z' in position ? position.z : 0 },
     overrides: {},
     parentId,
+    templateId: '',
   };
 }