Scene hierarchy: parentId with local positions, tree panel, reparenting and Parent node

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

src/runtime/World.ts

@@ -1,3 +1,4 @@
+import { worldPositionOf } from '../core/graph';
 import { findAsset, findScene, type Project } from '../core/model/Project';
 import { resolveSceneObjectProps } from '../core/properties/resolve';
 import type { Entity } from './Entity';
@@ -27,10 +28,11 @@ export class World {
     if (!scene) throw new Error(`Scene ${sceneId} not found`);
     const background = scene.props.backgroundColor;
     const world = new World(sceneId, typeof background === 'number' ? background : 0x000000);
+    const parentIds = new Map<Entity, string>();
     for (const object of scene.objects) {
       const asset = findAsset(project, object.assetId);
       if (!asset || asset.type === 'scene') continue;
-      world.entities.push({
+      const entity: Entity = {
         id: `e-${object.id}`,
         objectId: object.id,
         assetId: asset.id,
@@ -40,12 +42,25 @@ export class World {
         y: object.position.y,
         z: object.position.z,
         props: resolveSceneObjectProps(asset, object),
-        ownerObjectId: object.ownerId,
-      });
+        parent: null,
+      };
+      world.entities.push(entity);
+      if (object.parentId) parentIds.set(entity, object.parentId);
     }
+    // Link parents once every entity exists (storage order is not guaranteed to be parent-first).
+    for (const [entity, parentId] of parentIds) entity.parent = world.findByObjectId(parentId) ?? null;
     return world;
   }
 
+  /** World position of an entity: local position plus those of all ancestors. */
+  worldPosition(entity: Entity): { x: number; y: number; z: number } {
+    return worldPositionOf(entity);
+  }
+
+  childrenOf(entity: Entity): Entity[] {
+    return this.entities.filter((e) => e.parent === entity);
+  }
+
   addSystem(system: System): this {
     this.systems.push(system);
     system.start?.(this);
@@ -74,7 +89,9 @@ export class World {
     return this.entities.find((e) => e.type === type);
   }
 
+  /** Removes an entity and, recursively, everything parented to it. */
   remove(entity: Entity): void {
+    for (const child of this.childrenOf(entity)) this.remove(child);
     const index = this.entities.indexOf(entity);
     if (index >= 0) this.entities.splice(index, 1);
   }