Behaviors with Blueprint-style node graph, graph editor tab and BehaviorSystem

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

src/runtime/Entity.ts

@@ -17,5 +17,8 @@ export interface Entity {
   y: number;
   /** Draw order; 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;
 }