refactor(engine): 포션/진행 서비스 개선

- PotionService 로직 개선
- ProgressService 몬스터 등급 지원

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),