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/engine/progress_service.dart

@@ -984,12 +984,20 @@ class ProgressService {
     var updatedSkillSystem = skillSystem;
     var activeDoTs = [...combat.activeDoTs];
     var usedPotionTypes = {...combat.usedPotionTypes};
+    var activeDebuffs = [...combat.activeDebuffs];
     PotionInventory? updatedPotionInventory;
 
     // 새 전투 이벤트 수집
     final newEvents = <CombatEvent>[];
     final timestamp = updatedSkillSystem.elapsedMs;
 
+    // =========================================================================
+    // 만료된 디버프 정리
+    // =========================================================================
+    activeDebuffs = activeDebuffs
+        .where((debuff) => !debuff.isExpired(timestamp))
+        .toList();
+
     // =========================================================================
     // DOT 틱 처리
     // =========================================================================
@@ -1090,6 +1098,7 @@ class ProgressService {
         skillSystem: updatedSkillSystem,
         availableSkillIds: availableSkillIds,
         activeDoTs: activeDoTs,
+        activeDebuffs: activeDebuffs,
       );
 
       if (selectedSkill != null && selectedSkill.isAttack) {
@@ -1183,6 +1192,33 @@ class ProgressService {
             skillName: selectedSkill.name,
           ),
         );
+      } else if (selectedSkill != null && selectedSkill.isDebuff) {
+        // 디버프 스킬 사용
+        final skillResult = skillService.useDebuffSkill(
+          skill: selectedSkill,
+          player: playerStats,
+          skillSystem: updatedSkillSystem,
+          currentDebuffs: activeDebuffs,
+        );
+        playerStats = skillResult.updatedPlayer;
+        updatedSkillSystem = skillResult.updatedSkillSystem;
+
+        // 디버프 효과 추가 (기존 같은 디버프 제거 후)
+        if (skillResult.debuffEffect != null) {
+          activeDebuffs = activeDebuffs
+              .where((d) => d.effect.id != skillResult.debuffEffect!.effect.id)
+              .toList()
+            ..add(skillResult.debuffEffect!);
+        }
+
+        // 디버프 이벤트 생성
+        newEvents.add(
+          CombatEvent.playerDebuff(
+            timestamp: timestamp,
+            skillName: selectedSkill.name,
+            targetName: monsterStats.name,
+          ),
+        );
       } else {
         // 일반 공격
         final attackResult = calculator.playerAttackMonster(
@@ -1221,8 +1257,25 @@ class ProgressService {
     // 몬스터가 살아있으면 반격
     if (monsterStats.isAlive &&
         monsterAccumulator >= monsterStats.attackDelayMs) {
+      // 디버프 효과 적용된 몬스터 스탯 계산
+      var debuffedMonster = monsterStats;
+      if (activeDebuffs.isNotEmpty) {
+        double atkMod = 0;
+        for (final debuff in activeDebuffs) {
+          if (!debuff.isExpired(timestamp)) {
+            atkMod += debuff.effect.atkModifier; // 음수 값
+          }
+        }
+        // ATK 감소 적용 (최소 10% ATK 유지)
+        final newAtk = (monsterStats.atk * (1 + atkMod)).round().clamp(
+          monsterStats.atk ~/ 10,
+          monsterStats.atk,
+        );
+        debuffedMonster = monsterStats.copyWith(atk: newAtk);
+      }
+
       final attackResult = calculator.monsterAttackPlayer(
-        attacker: monsterStats,
+        attacker: debuffedMonster,
         defender: playerStats,
       );
       playerStats = attackResult.updatedDefender;
@@ -1288,6 +1341,7 @@ class ProgressService {
         recentEvents: recentEvents,
         activeDoTs: activeDoTs,
         usedPotionTypes: usedPotionTypes,
+        activeDebuffs: activeDebuffs,
       ),
       skillSystem: updatedSkillSystem,
       potionInventory: updatedPotionInventory,