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

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

tests/core/world-sync.test.ts

@@ -82,6 +82,41 @@ describe('world ↔ model synchronisation (editing while playing)', () => {
     expect(sceneOf().objects.map((o) => o.id)).toEqual([a]);
   });
 
+  it('spawn adds an object to the runtime scene and an entity at once, without resetting others', () => {
+    const { project, sceneId, spriteId, a, sceneOf } = setup();
+    const world = World.instantiate(project, sceneId);
+    const entityA = world.findByObjectId(a)!;
+    entityA.x = 77; // moved earlier in this tick – a spawn must not pull the model value back
+
+    const child = world.spawn(spriteId, { x: 10, y: 20, z: 1 }, entityA)!;
+    expect(child).not.toBeNull();
+    expect(child.parent).toBe(entityA);
+    expect(child.type).toBe('sprite');
+    expect(child.name).toBe('Box');
+    expect(child.props.width).toBe(100);
+    expect(entityA.x).toBe(77);
+    const object = sceneOf().objects.find((o) => o.id === child.objectId)!;
+    expect(object.parentId).toBe(a);
+    expect(object.position).toEqual({ x: 10, y: 20, z: 1 });
+
+    // The spawn is a structural change the panels must see – reported once.
+    expect(world.syncToModel()).toBe(true);
+    expect(world.syncToModel()).toBe(false);
+    // A full sync from the model keeps the spawned entity (identity included).
+    world.syncFromModel();
+    expect(world.findByObjectId(child.objectId)).toBe(child);
+    expect(child.parent).toBe(entityA);
+
+    // Destroying the spawned entity drops its object again.
+    world.remove(child);
+    expect(world.syncToModel()).toBe(true);
+    expect(sceneOf().objects.some((o) => o.id === child.objectId)).toBe(false);
+
+    // Unknown or unplaceable assets spawn nothing.
+    expect(world.spawn('nope', { x: 0, y: 0, z: 0 }, null)).toBeNull();
+    expect(world.spawn(sceneId, { x: 0, y: 0, z: 0 }, null)).toBeNull();
+  });
+
   it('a play-mode copy leaves the original model untouched', () => {
     const { project, sceneId, a, object } = setup();
     const copy = deserializeProject(serializeProject(project), project.id);