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

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

src/core/model/container.ts

@@ -0,0 +1,28 @@
+import { newId } from '../ids';
+import type { ContainerAsset } from './Asset';
+import { findAsset, type Project } from './Project';
+import type { SceneObject } from './SceneObject';
+
+/** Nesting depth at which a container inside a container inside … stops being expanded. */
+const MAX_NESTING = 8;
+
+/**
+ * The scene objects a placed container consists of: one fresh object per template object,
+ * parented below `containerObjectId` (or below the copy of their template parent), each
+ * remembering its template id. A template object that is itself a container is expanded in
+ * turn – unless the template already spells its content out (that is what a container built
+ * from a selection containing another container does).
+ */
+export function instantiateContainer(project: Project, asset: ContainerAsset, containerObjectId: string, depth = 0): SceneObject[] {
+  const ids = new Map(asset.objects.map((t) => [t.id, newId()]));
+  const result: SceneObject[] = [];
+  for (const template of asset.objects) {
+    const id = ids.get(template.id)!;
+    const parentId = template.parentId ? ids.get(template.parentId) ?? containerObjectId : containerObjectId;
+    result.push({ id, assetId: template.assetId, name: template.name, position: { ...template.position }, overrides: { ...template.overrides }, parentId, templateId: template.id });
+    const nested = findAsset(project, template.assetId);
+    const hasExplicitContent = asset.objects.some((o) => o.parentId === template.id);
+    if (nested?.type === 'container' && !hasExplicitContent && depth < MAX_NESTING) result.push(...instantiateContainer(project, nested, id, depth + 1));
+  }
+  return result;
+}