feat: tutorial step state machine with persistence

Adds TutorialNotifier (dragPiece → clearLine → explainHud → null) backed
by SaveRepository.markTutorialDone(); out-of-order events are silently
ignored. Registers tutorialProvider in providers.dart.

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

lib/state/providers.dart

@@ -9,6 +9,7 @@ import 'game_session_notifier.dart';
 import 'progress_notifier.dart';
 import 'season_flow_notifier.dart';
 import 'streak_notifier.dart';
+import 'tutorial_notifier.dart';
 
 final gameSessionProvider =
     NotifierProvider<GameSessionNotifier, GameViewState?>(
@@ -46,6 +47,10 @@ final streakProvider = NotifierProvider<StreakNotifier, StreakState>(
   StreakNotifier.new,
 );
 
+final tutorialProvider = NotifierProvider<TutorialNotifier, TutorialStep?>(
+  TutorialNotifier.new,
+);
+
 /// The visual theme of whatever season is in play; fallback outside seasons
 /// (home, endless). Pure model — UI converts via ThemeColors.
 final activeThemeProvider = Provider<SeasonTheme>((ref) {