refactor(engine): 포션/진행 서비스 개선

- PotionService 로직 개선
- ProgressService 몬스터 등급 지원

lib/src/core/engine/potion_service.dart

@@ -1,4 +1,7 @@
+import 'dart:math' as math;
+
 import 'package:asciineverdie/data/potion_data.dart';
+import 'package:asciineverdie/src/core/model/monster_grade.dart';
 import 'package:asciineverdie/src/core/model/potion.dart';
 
 /// 물약 서비스
@@ -357,19 +360,35 @@ class PotionService {
   ///
   /// 전투 승리 시 물약 드랍 여부 결정 및 물약 획득
   /// [playerLevel] 플레이어 레벨 (드랍 확률 및 티어 결정)
+  /// [monsterLevel] 몬스터 레벨 (티어 결정에 영향)
+  /// [monsterGrade] 몬스터 등급 (드랍 확률 보너스)
   /// [inventory] 현재 물약 인벤토리
   /// [roll] 0~99 범위의 난수 (드랍 확률 판정)
   /// [typeRoll] 0~99 범위의 난수 (HP/MP 결정)
   /// Returns: (업데이트된 인벤토리, 드랍된 물약 또는 null)
   (PotionInventory, Potion?) tryPotionDrop({
     required int playerLevel,
+    required int monsterLevel,
+    required MonsterGrade monsterGrade,
     required PotionInventory inventory,
     required int roll,
     required int typeRoll,
   }) {
-    // 드랍 확률 계산
+    // 기본 드랍 확률 계산
-    final dropChance = (baseDropChance + playerLevel * dropChancePerLevel)
+    var dropChance = (baseDropChance + playerLevel * dropChancePerLevel)
         .clamp(baseDropChance, maxDropChance);
+
+    // 몬스터 등급 보너스 (Elite +5%, Boss +15%)
+    dropChance += monsterGrade.potionDropBonus;
+
+    // 몬스터 레벨 > 플레이어 레벨이면 추가 확률 (+1%/레벨 차이)
+    if (monsterLevel > playerLevel) {
+      dropChance += (monsterLevel - playerLevel) * 0.01;
+    }
+
+    // 최대 확률 제한 (50%)
+    dropChance = dropChance.clamp(0.0, 0.5);
+
     final dropThreshold = (dropChance * 100).round();
 
     // 드랍 실패
@@ -380,8 +399,9 @@ class PotionService {
     // 물약 타입 결정 (60% HP, 40% MP)
     final isHpPotion = typeRoll < 60;
 
-    // 레벨 기반 티어 결정
+    // 티어 결정: max(플레이어 레벨, 몬스터 레벨) 기반
-    final tier = PotionData.tierForLevel(playerLevel);
+    final effectiveLevel = math.max(playerLevel, monsterLevel);
+    final tier = PotionData.tierForLevel(effectiveLevel);
 
     // 물약 선택
     final Potion? potion;

lib/src/core/engine/progress_service.dart

@@ -14,6 +14,7 @@ import 'package:asciineverdie/src/core/model/equipment_item.dart';
 import 'package:asciineverdie/src/core/model/equipment_slot.dart';
 import 'package:asciineverdie/src/core/model/game_state.dart';
 import 'package:asciineverdie/src/core/model/monster_combat_stats.dart';
+import 'package:asciineverdie/src/core/model/monster_grade.dart';
 import 'package:asciineverdie/src/core/model/potion.dart';
 import 'package:asciineverdie/src/core/model/pq_config.dart';
 import 'package:asciineverdie/src/core/model/skill.dart';
@@ -591,6 +592,7 @@ class ProgressService {
         monsterBaseName: monsterResult.baseName,
         monsterPart: monsterResult.part,
         monsterLevel: monsterResult.level,
+        monsterGrade: monsterResult.grade,
       ),
       currentCombat: combatState,
     );
@@ -646,6 +648,7 @@ class ProgressService {
         monsterBaseName: 'Glitch God',
         monsterPart: '*', // 특수 전리품
         monsterLevel: glitchGod.level,
+        monsterGrade: MonsterGrade.boss, // 최종 보스는 항상 boss 등급
       ),
       currentCombat: combatState,
     );
@@ -963,8 +966,12 @@ class ProgressService {
     // 물약 드랍 시도
     final potionService = const PotionService();
     final rng = resultState.rng;
+    final monsterLevel = taskInfo.monsterLevel ?? resultState.traits.level;
+    final monsterGrade = taskInfo.monsterGrade ?? MonsterGrade.normal;
     final (updatedPotionInventory, droppedPotion) = potionService.tryPotionDrop(
       playerLevel: resultState.traits.level,
+      monsterLevel: monsterLevel,
+      monsterGrade: monsterGrade,
       inventory: resultState.potionInventory,
       roll: rng.nextInt(100),
       typeRoll: rng.nextInt(100),