Fix stop button - track and stop all active audio nodes

- Track all oscillators/sources in APU activeNodes Set
- Add stopAll() method that immediately stops all tracked nodes
- Auto-remove nodes from tracking when they end naturally
- GameBoyPlayer.stop() now calls apu.stopAll()

src-v2/core/GameBoyPlayer.ts

@@ -230,7 +230,7 @@ export class GameBoyPlayer {
    */
   stop(): void {
     this.isPlaying = false;
-    this.apu.reset();
+    this.apu.stopAll();
     console.log('Playback stopped');
   }