fix: wait for video metadata before playing and set onEnd before play

- Reorder onEnd callback before play() in Engine.goToScene to prevent
  missed ended event if video ends synchronously
- Wait for loadedmetadata event in VideoManager.play() before seeking
  to ensure currentTime reset works correctly on new video sources

engine/core/Engine.ts

@@ -46,13 +46,13 @@ export class Engine {
       this.stateManager.apply(scene.onEnter)
     }
 
-    this.videoManager.play(scene.videoUrl)
-    this.emit('sceneChange', scene)
-
     this.videoManager.onEnd(() => {
       this.emit('videoEnd', scene)
       this.onVideoEnd(scene)
     })
+
+    this.videoManager.play(scene.videoUrl)
+    this.emit('sceneChange', scene)
   }
 
   private onVideoEnd(scene: SceneNode) {