feat(ads): stage-end interstitial gated by policy; restart resets round

lib/state/game_session_notifier.dart

@@ -75,6 +75,7 @@ class GameSessionNotifier extends Notifier<GameViewState?> {
     final stage = _stage;
     if (stage == null) throw StateError('no stage to restart');
     startStage(stage, attempt: _attempt + 1, generator: _generatorOverride);
+    ref.read(adServiceProvider).onRoundStart();
     if (stage.endless) {
       ref.read(analyticsProvider).endlessStart();
     } else {

lib/ui/screens/game_screen.dart

@@ -382,8 +382,12 @@ class _GameScreenState extends ConsumerState<GameScreen>
           [
             if (flow != null && flow.hasNext)
               FilledButton(
-                onPressed:
-                    ref.read(seasonFlowProvider.notifier).nextStage,
+                onPressed: () {
+                  ref.read(seasonFlowProvider.notifier).nextStage();
+                  if (!view.endless) {
+                    ref.read(adServiceProvider).maybeShowInterstitial();
+                  }
+                },
                 child: Text(l10n.nextStage),
               ),
             TextButton(
@@ -460,7 +464,10 @@ class _GameScreenState extends ConsumerState<GameScreen>
           l10n.stageFailed,
           [
             FilledButton(
-              onPressed: notifier.restart,
+              onPressed: () {
+                ref.read(adServiceProvider).maybeShowInterstitial();
+                notifier.restart();
+              },
               child: Text(l10n.playAgain),
             ),
           ],