fix: clear season flow when starting Classic so tutorial/theme can't leak

- Add SeasonFlowNotifier.clear() to null out the flow state
- Call clear() in HomeScreen's Classic button before startStage()
- Broaden tutorial-end guard to next.phase != GamePhase.playing (covers stuck)
- Add regression test: clear() resets flow to null

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

lib/state/season_flow_notifier.dart

@@ -41,4 +41,8 @@ class SeasonFlowNotifier extends Notifier<SeasonFlow?> {
     if (flow == null || !flow.hasNext) return;
     startSeasonStage(flow.pack, flow.index + 1);
   }
+
+  /// Leaving season play (e.g. starting a Classic run) clears the flow so
+  /// stale stage context can't leak into other modes.
+  void clear() => state = null;
 }