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

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

src/runtime/World.ts

@@ -1,4 +1,5 @@
 import { worldPositionOf } from '../core/graph';
+import { instantiateContainer } from '../core/model/container';
 import { newId } from '../core/ids';
 import type { Asset } from '../core/model/Asset';
 import { isPlaceableType } from '../core/model/AssetTypes';
@@ -96,9 +97,16 @@ export class World {
     if (!scene || !asset || !isPlaceableType(asset.type)) return null;
     const parentEntity = parent && this.entities.includes(parent) ? parent : null;
     const object = createSceneObject(newId(), asset.id, position, parentEntity?.objectId ?? '');
-    scene.objects.push(object);
+    // A container brings its content along (parented below it, in template order).
+    const created = [object, ...(asset.type === 'container' ? instantiateContainer(this.project, asset, object.id) : [])];
+    scene.objects.push(...created);
     const entity = this.materialize(object, asset);
     entity.parent = parentEntity;
+    for (const child of created.slice(1)) {
+      const childAsset = findAsset(this.project, child.assetId);
+      if (!childAsset || childAsset.type === 'scene') continue;
+      this.materialize(child, childAsset).parent = this.findByObjectId(child.parentId) ?? null;
+    }
     this.structureChanged = true;
     return entity;
   }