feat(combat): 디버프 시스템 추가

- CombatEventType.playerDebuff 추가
- CombatState에 activeDebuffs 목록 추가
- SkillService.useDebuffSkill() 구현
- 스킬 자동 선택에 디버프 우선순위 추가
- 밸런스 상수 업데이트

lib/src/core/model/combat_event.dart

@@ -27,6 +27,9 @@ enum CombatEventType {
   /// 플레이어 버프
   playerBuff,
 
+  /// 플레이어 디버프 (적에게 적용)
+  playerDebuff,
+
   /// DOT 틱 데미지
   dotTick,
 
@@ -209,6 +212,20 @@ class CombatEvent {
     );
   }
 
+  /// 디버프 이벤트 생성 (적에게 디버프 적용)
+  factory CombatEvent.playerDebuff({
+    required int timestamp,
+    required String skillName,
+    required String targetName,
+  }) {
+    return CombatEvent(
+      type: CombatEventType.playerDebuff,
+      timestamp: timestamp,
+      skillName: skillName,
+      targetName: targetName,
+    );
+  }
+
   /// DOT 틱 이벤트 생성
   factory CombatEvent.dotTick({
     required int timestamp,

lib/src/core/model/combat_state.dart

@@ -21,6 +21,7 @@ class CombatState {
     this.recentEvents = const [],
     this.activeDoTs = const [],
     this.usedPotionTypes = const {},
+    this.activeDebuffs = const [],
   });
 
   /// 플레이어 전투 스탯
@@ -56,6 +57,9 @@ class CombatState {
   /// 이번 전투에서 사용한 물약 종류 (종류별 1회 제한)
   final Set<PotionType> usedPotionTypes;
 
+  /// 몬스터에 적용된 활성 디버프 목록
+  final List<ActiveBuff> activeDebuffs;
+
   // ============================================================================
   // 유틸리티
   // ============================================================================
@@ -88,6 +92,24 @@ class CombatState {
     });
   }
 
+  /// 활성 디버프 존재 여부
+  bool get hasActiveDebuffs => activeDebuffs.isNotEmpty;
+
+  /// 몬스터에 적용된 총 디버프 효과 계산
+  ///
+  /// 디버프 효과는 몬스터 ATK/DEF에 부정적 배율로 적용됨
+  ({double atkMod, double defMod}) get totalDebuffModifiers {
+    double atkMod = 0;
+    double defMod = 0;
+
+    for (final debuff in activeDebuffs) {
+      atkMod += debuff.effect.atkModifier;
+      defMod += debuff.effect.defModifier;
+    }
+
+    return (atkMod: atkMod, defMod: defMod);
+  }
+
   CombatState copyWith({
     CombatStats? playerStats,
     MonsterCombatStats? monsterStats,
@@ -100,6 +122,7 @@ class CombatState {
     List<CombatEvent>? recentEvents,
     List<DotEffect>? activeDoTs,
     Set<PotionType>? usedPotionTypes,
+    List<ActiveBuff>? activeDebuffs,
   }) {
     return CombatState(
       playerStats: playerStats ?? this.playerStats,
@@ -115,6 +138,7 @@ class CombatState {
       recentEvents: recentEvents ?? this.recentEvents,
       activeDoTs: activeDoTs ?? this.activeDoTs,
       usedPotionTypes: usedPotionTypes ?? this.usedPotionTypes,
+      activeDebuffs: activeDebuffs ?? this.activeDebuffs,
     );
   }