fix: stabilize IPTV playback and implement request throttling

- Add playback lock to XtreamService to delay EPG during stream init
- Increase MPEG-TS stash buffer to 512KB for better stability
- Enable liveBufferLatencyChasing to prevent stream drift
- Tune HLS parameters for balanced speed and reliability

lib/features/iptv/screens/player_screen.dart

@@ -140,6 +140,12 @@ class _PlayerScreenState extends ConsumerState<PlayerScreen> {
         streamUrl = service.getLiveStreamUrlTs(currentStreamId);
       }
 
+      // STABILIZATION: Lock background requests during stream init
+      service.isPlaybackLoading = true;
+      Future.delayed(const Duration(seconds: 10), () {
+        service.isPlaybackLoading = false;
+      });
+
       _currentStreamUrl = streamUrl;
 
       final encodedUrl = Uri.encodeComponent(streamUrl);