feat(story): Phase 9 스토리/엔딩 시스템 구현

- story_data.dart: 5개 Act 스토리 텍스트 및 ASCII 아트
- story_service.dart: Act 전환/보스 조우/엔딩 이벤트 관리
- cinematic_view.dart: 풀스크린 시네마틱 UI (페이드, 스킵)
- game_play_screen.dart: 레벨 기반 Act 전환 시 시네마틱 재생

lib/src/core/engine/story_service.dart

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+import 'dart:async';
+
+import 'package:askiineverdie/data/story_data.dart';
+
+/// 스토리 이벤트 타입 (Story Event Type)
+enum StoryEventType {
+  actStart, // Act 시작
+  actComplete, // Act 완료
+  bossEncounter, // 보스 조우
+  bossDefeat, // 보스 처치
+  ending, // 엔딩
+}
+
+/// 스토리 이벤트 (Story Event)
+class StoryEvent {
+  const StoryEvent({
+    required this.type,
+    required this.act,
+    this.data,
+  });
+
+  final StoryEventType type;
+  final StoryAct act;
+  final Map<String, dynamic>? data;
+}
+
+/// 스토리 서비스 (Phase 9: Story Progression Management)
+///
+/// Act 전환, 시네마틱 트리거, 보스 조우 관리
+class StoryService {
+  StoryService();
+
+  final _eventController = StreamController<StoryEvent>.broadcast();
+
+  /// 스토리 이벤트 스트림 (Story Event Stream)
+  Stream<StoryEvent> get events => _eventController.stream;
+
+  // 현재 Act 추적
+  StoryAct _currentAct = StoryAct.prologue;
+  bool _hasSeenPrologue = false;
+  final Set<StoryAct> _completedActs = {};
+
+  /// 현재 Act (Current Act)
+  StoryAct get currentAct => _currentAct;
+
+  /// 프롤로그 시청 여부
+  bool get hasSeenPrologue => _hasSeenPrologue;
+
+  /// 완료된 Act 목록
+  Set<StoryAct> get completedActs => Set.unmodifiable(_completedActs);
+
+  /// 레벨 변화 감지 및 Act 전환 처리 (Process Level Change)
+  ///
+  /// 레벨업 시 호출하여 Act 전환 이벤트 트리거
+  StoryEvent? processLevelChange(int oldLevel, int newLevel) {
+    final oldAct = getActForLevel(oldLevel);
+    final newAct = getActForLevel(newLevel);
+
+    // 새 게임 시작 (프롤로그)
+    if (oldLevel == 0 && newLevel == 1 && !_hasSeenPrologue) {
+      _hasSeenPrologue = true;
+      _currentAct = StoryAct.prologue;
+      final event = StoryEvent(
+        type: StoryEventType.actStart,
+        act: StoryAct.prologue,
+      );
+      _eventController.add(event);
+      return event;
+    }
+
+    // Act 전환 감지
+    if (newAct != oldAct && newAct != _currentAct) {
+      // 이전 Act 완료 처리
+      if (_currentAct != StoryAct.prologue) {
+        _completedActs.add(_currentAct);
+        _eventController.add(StoryEvent(
+          type: StoryEventType.actComplete,
+          act: _currentAct,
+        ));
+      }
+
+      // 새 Act 시작
+      _currentAct = newAct;
+      final event = StoryEvent(
+        type: StoryEventType.actStart,
+        act: newAct,
+      );
+      _eventController.add(event);
+      return event;
+    }
+
+    return null;
+  }
+
+  /// 보스 조우 처리 (Process Boss Encounter)
+  void processBossEncounter(String monsterName) {
+    // BOSS: 접두사가 있는 몬스터인지 확인
+    if (!monsterName.startsWith('BOSS:')) return;
+
+    final event = StoryEvent(
+      type: StoryEventType.bossEncounter,
+      act: _currentAct,
+      data: {'bossName': monsterName},
+    );
+    _eventController.add(event);
+  }
+
+  /// 보스 처치 처리 (Process Boss Defeat)
+  void processBossDefeat(String monsterName) {
+    if (!monsterName.startsWith('BOSS:')) return;
+
+    final event = StoryEvent(
+      type: StoryEventType.bossDefeat,
+      act: _currentAct,
+      data: {'bossName': monsterName},
+    );
+    _eventController.add(event);
+
+    // 최종 보스 처치 시 엔딩
+    if (monsterName.contains('Primordial Glitch')) {
+      _triggerEnding();
+    }
+  }
+
+  /// 엔딩 트리거 (Trigger Ending)
+  void _triggerEnding() {
+    _completedActs.add(StoryAct.act5);
+    _currentAct = StoryAct.ending;
+    _eventController.add(StoryEvent(
+      type: StoryEventType.ending,
+      act: StoryAct.ending,
+    ));
+  }
+
+  /// 시네마틱 데이터 가져오기 (Get Cinematic Data)
+  List<CinematicStep> getCinematicSteps(StoryAct act) {
+    return cinematicData[act] ?? [];
+  }
+
+  /// Act 제목 가져오기 (Get Act Title)
+  String getActTitle(StoryAct act) {
+    return actTitles[act] ?? 'Unknown';
+  }
+
+  /// Act 보스 이름 가져오기 (Get Act Boss Name)
+  String? getActBossName(StoryAct act) {
+    return actBossNames[act];
+  }
+
+  /// 저장 데이터로 상태 복원 (Restore State from Save)
+  void restoreState({
+    required StoryAct currentAct,
+    required bool hasSeenPrologue,
+    required Set<StoryAct> completedActs,
+  }) {
+    _currentAct = currentAct;
+    _hasSeenPrologue = hasSeenPrologue;
+    _completedActs
+      ..clear()
+      ..addAll(completedActs);
+  }
+
+  /// 현재 상태를 저장 데이터로 변환 (Convert to Save Data)
+  Map<String, dynamic> toSaveData() {
+    return {
+      'currentAct': _currentAct.index,
+      'hasSeenPrologue': _hasSeenPrologue,
+      'completedActs': _completedActs.map((a) => a.index).toList(),
+    };
+  }
+
+  /// 저장 데이터에서 복원 (Restore from Save Data)
+  void fromSaveData(Map<String, dynamic> data) {
+    _currentAct = StoryAct.values[data['currentAct'] as int? ?? 0];
+    _hasSeenPrologue = data['hasSeenPrologue'] as bool? ?? false;
+    final completedIndices = data['completedActs'] as List<dynamic>? ?? [];
+    _completedActs
+      ..clear()
+      ..addAll(completedIndices.map((i) => StoryAct.values[i as int]));
+  }
+
+  /// 서비스 정리 (Dispose)
+  void dispose() {
+    _eventController.close();
+  }
+}