feat(core): 엔진, 모델, 애니메이션 개선

- ProgressService 로직 개선
- CombatCalculator 업데이트
- GameState, MonsterCombatStats 확장
- CanvasBattleComposer 개선

lib/src/core/model/game_state.dart

@@ -475,7 +475,8 @@ class Inventory {
   final int gold;
   final List<InventoryEntry> items;
 
-  factory Inventory.empty() => const Inventory(gold: 0, items: []);
+  /// 초기 골드 1000 지급 (캐릭터 생성 시)
+  factory Inventory.empty() => const Inventory(gold: 1000, items: []);
 
   Inventory copyWith({int? gold, List<InventoryEntry>? items}) {
     return Inventory(gold: gold ?? this.gold, items: items ?? this.items);
@@ -800,6 +801,7 @@ class ProgressState {
     this.deathCount = 0,
     this.finalBossState = FinalBossState.notSpawned,
     this.pendingActCompletion = false,
+    this.bossLevelingEndTime,
   });
 
   final ProgressBarState task;
@@ -835,6 +837,10 @@ class ProgressState {
   /// Act Boss 처치 대기 중 여부 (처치 후 시네마틱 재생 트리거)
   final bool pendingActCompletion;
 
+  /// 보스 사망 후 레벨링 모드 종료 시간 (milliseconds since epoch)
+  /// null이면 레벨링 모드 아님, 값이 있으면 해당 시간까지 레벨링
+  final int? bossLevelingEndTime;
+
   factory ProgressState.empty() => ProgressState(
     task: ProgressBarState.empty(),
     quest: ProgressBarState.empty(),
@@ -867,6 +873,8 @@ class ProgressState {
     int? deathCount,
     FinalBossState? finalBossState,
     bool? pendingActCompletion,
+    int? bossLevelingEndTime,
+    bool clearBossLevelingEndTime = false,
   }) {
     return ProgressState(
       task: task ?? this.task,
@@ -885,8 +893,17 @@ class ProgressState {
       deathCount: deathCount ?? this.deathCount,
       finalBossState: finalBossState ?? this.finalBossState,
       pendingActCompletion: pendingActCompletion ?? this.pendingActCompletion,
+      bossLevelingEndTime: clearBossLevelingEndTime
+          ? null
+          : (bossLevelingEndTime ?? this.bossLevelingEndTime),
     );
   }
+
+  /// 현재 레벨링 모드인지 확인
+  bool get isInBossLevelingMode {
+    if (bossLevelingEndTime == null) return false;
+    return DateTime.now().millisecondsSinceEpoch < bossLevelingEndTime!;
+  }
 }
 
 class QueueEntry {