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

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

src/core/model/SceneObject.ts

@@ -10,24 +10,28 @@ export interface SceneObject {
   assetId: string;
   /** Optional display name; falls back to the asset name when empty. */
   name: string;
-  /** World position plus draw order (`z`). */
+  /**
+   * Position **relative to the parent** (world position when there is no parent) plus the
+   * draw order `z`, which is relative as well. See `hierarchy.ts` for world coordinates.
+   */
   position: Position;
   overrides: PropertyBag;
   /**
-   * Another object of the same scene this one acts on (used by behaviors: "the owner").
-   * Empty string = none. Any object may carry one, only behaviors interpret it.
+   * Parent object of the same scene, empty string = top level. Children move with their
+   * parent and are removed with it. Behaviors treat their parent as the "owner" the graph
+   * acts on.
    */
-  ownerId: string;
+  parentId: string;
 }
 
 /** Creates a scene object; `z` defaults to 0 when the caller only has a 2D point. */
-export function createSceneObject(id: string, assetId: string, position: Vec2 | Position): SceneObject {
+export function createSceneObject(id: string, assetId: string, position: Vec2 | Position, parentId = ''): SceneObject {
   return {
     id,
     assetId,
     name: '',
     position: { x: position.x, y: position.y, z: 'z' in position ? position.z : 0 },
     overrides: {},
-    ownerId: '',
+    parentId,
   };
 }