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) {

lib/state/tutorial_notifier.dart

@@ -0,0 +1,40 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import 'providers.dart';
+
+enum TutorialStep { dragPiece, clearLine, explainHud }
+
+/// First-play guided tutorial. State null = inactive. Events arriving in the
+/// wrong step are ignored, so engine wiring can fire them unconditionally.
+class TutorialNotifier extends Notifier<TutorialStep?> {
+  @override
+  TutorialStep? build() => null;
+
+  void start() {
+    if (ref.read(saveRepositoryProvider).tutorialDone) return;
+    state = TutorialStep.dragPiece;
+  }
+
+  void onPlaced() {
+    if (state == TutorialStep.dragPiece) state = TutorialStep.clearLine;
+  }
+
+  void onLineCleared() {
+    if (state == TutorialStep.clearLine) state = TutorialStep.explainHud;
+  }
+
+  Future<void> dismissHud() async {
+    if (state != TutorialStep.explainHud) return;
+    await _finish();
+  }
+
+  Future<void> skip() async {
+    if (state == null) return;
+    await _finish();
+  }
+
+  Future<void> _finish() async {
+    state = null;
+    await ref.read(saveRepositoryProvider).markTutorialDone();
+  }
+}