feat(core): 몬스터 등급 시스템 추가

- MonsterGrade 열거형 및 색상 정의
- GameState/ItemStats 확장
- pq_logic 유틸리티 함수 추가
- ASCII 색상 상수 추가

lib/src/core/model/game_state.dart

@@ -5,6 +5,7 @@ import 'package:asciineverdie/src/core/model/combat_state.dart';
 import 'package:asciineverdie/src/core/model/equipment_item.dart';
 import 'package:asciineverdie/src/core/model/equipment_slot.dart';
 import 'package:asciineverdie/src/core/model/item_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/skill.dart';
 import 'package:asciineverdie/src/core/util/deterministic_random.dart';
@@ -280,6 +281,7 @@ class TaskInfo {
     this.monsterBaseName,
     this.monsterPart,
     this.monsterLevel,
+    this.monsterGrade,
   });
 
   final String caption;
@@ -294,6 +296,9 @@ class TaskInfo {
   /// 킬 태스크의 몬스터 레벨 (애니메이션 크기 결정용)
   final int? monsterLevel;
 
+  /// 킬 태스크의 몬스터 등급 (Normal/Elite/Boss)
+  final MonsterGrade? monsterGrade;
+
   factory TaskInfo.empty() =>
       const TaskInfo(caption: '', type: TaskType.neutral);
 
@@ -303,6 +308,7 @@ class TaskInfo {
     String? monsterBaseName,
     String? monsterPart,
     int? monsterLevel,
+    MonsterGrade? monsterGrade,
   }) {
     return TaskInfo(
       caption: caption ?? this.caption,
@@ -310,6 +316,7 @@ class TaskInfo {
       monsterBaseName: monsterBaseName ?? this.monsterBaseName,
       monsterPart: monsterPart ?? this.monsterPart,
       monsterLevel: monsterLevel ?? this.monsterLevel,
+      monsterGrade: monsterGrade ?? this.monsterGrade,
     );
   }
 }