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

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

src/runtime/Entity.ts

@@ -13,12 +13,13 @@ export interface Entity {
   assetId: string;
   type: AssetType;
   name: string;
+  /** Position local to `parent` (world position for top-level entities). */
   x: number;
   y: number;
-  /** Draw order; higher is drawn on top. */
+  /** Draw order, relative to the parent as well; higher is drawn on top. */
   z: number;
   /** For behaviors: the resolved variables (mutable state). For others: resolved properties. */
   props: PropertyBag;
-  /** Scene object id of the owner (behaviors), empty when none. */
-  ownerObjectId: string;
+  /** Parent entity (mirrors `SceneObject.parentId`); children move and die with it. */
+  parent: Entity | null;
 }