fix(ads): reset ad round in startStage so nextStage advance counts (review fix)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/ui/screens/game_screen.dart

@@ -117,7 +117,6 @@ class _GameScreenState extends ConsumerState<GameScreen>
 
   void _onSessionChange(GameViewState? prev, GameViewState? next) {
     if (next == null) return;
-    if (prev == null) ref.read(adServiceProvider).onRoundStart();
     final audio = ref.read(audioServiceProvider);
     if (prev?.fxTick != next.fxTick && next.lastPlacement != null) {
       final placement = next.lastPlacement!;