Spawn node: place assets at runtime into the play copy, Spawner demo behavior

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

src/core/graph/NodeRegistry.ts

@@ -62,6 +62,12 @@ export interface NodeContext {
    */
   moveAndSlide(target: EntityHandle, dx: number, dy: number): { blockedX: boolean; blockedY: boolean };
   log(message: string): void;
+  /**
+   * Places a new instance of an asset into the running scene and returns it. The position is
+   * local to `parent` (world when none). Returns null when the asset does not exist or cannot
+   * be placed (a scene). Spawned behaviors run their `On start` on the next tick.
+   */
+  spawn(assetId: string, position: { x: number; y: number; z: number }, parent: EntityHandle | null): EntityHandle | null;
   destroy(target: EntityHandle): void;
   stop(): void;
 }