refactor(model): freezed 패키지 도입으로 보일러플레이트 제거

- ItemStats, CombatStats, EquipmentItem을 freezed로 마이그레이션
- copyWith, toJson/fromJson 자동 생성
- 세이브 파일 호환성 유지

lib/src/core/model/equipment_item.dart

@@ -1,51 +1,38 @@
+import 'package:freezed_annotation/freezed_annotation.dart';
+
 import 'package:asciineverdie/src/core/model/equipment_slot.dart';
 import 'package:asciineverdie/src/core/model/item_stats.dart';
 
+part 'equipment_item.freezed.dart';
+part 'equipment_item.g.dart';
+
 /// 장비 아이템
 ///
 /// 개별 장비의 이름, 슬롯, 레벨, 스탯 등을 포함하는 클래스.
 /// 불변(immutable) 객체로 설계됨.
-class EquipmentItem {
-  const EquipmentItem({
-    required this.name,
-    required this.slot,
-    required this.level,
-    required this.weight,
-    required this.stats,
-    required this.rarity,
-  });
+@freezed
+class EquipmentItem with _$EquipmentItem {
+  const EquipmentItem._();
 
-  /// 아이템 이름 (예: "Flaming Sword of Doom")
-  final String name;
+  const factory EquipmentItem({
+    /// 아이템 이름 (예: "Flaming Sword of Doom")
+    required String name,
+    /// 장착 슬롯
+    @JsonKey(fromJson: _slotFromJson, toJson: _slotToJson)
+    required EquipmentSlot slot,
+    /// 아이템 레벨
+    required int level,
+    /// 무게 (STR 기반 휴대 제한용)
+    required int weight,
+    /// 아이템 스탯 보정치
+    required ItemStats stats,
+    /// 희귀도
+    @JsonKey(fromJson: _rarityFromJson, toJson: _rarityToJson)
+    required ItemRarity rarity,
+  }) = _EquipmentItem;
 
-  /// 장착 슬롯
-  final EquipmentSlot slot;
-
-  /// 아이템 레벨
-  final int level;
-
-  /// 무게 (STR 기반 휴대 제한용)
-  final int weight;
-
-  /// 아이템 스탯 보정치
-  final ItemStats stats;
-
-  /// 희귀도
-  final ItemRarity rarity;
-
-  /// 가중치 (자동 장착 비교용)
-  ///
-  /// 가중치 = 기본값 + (레벨 * 10) + (희귀도 보너스) + (스탯 합계)
-  int get itemWeight {
-    const baseValue = 10;
-    return baseValue + (level * 10) + rarity.weightBonus + stats.totalStatValue;
-  }
-
-  /// 빈 아이템 여부
-  bool get isEmpty => name.isEmpty;
-
-  /// 유효한 아이템 여부
-  bool get isNotEmpty => name.isNotEmpty;
+  factory EquipmentItem.fromJson(Map<String, dynamic> json) =>
+      _$EquipmentItemFromJson(json);
 
   /// 빈 아이템 생성 (특정 슬롯)
   factory EquipmentItem.empty(EquipmentSlot slot) {
@@ -54,7 +41,7 @@ class EquipmentItem {
       slot: slot,
       level: 0,
       weight: 0,
-      stats: ItemStats.empty,
+      stats: const ItemStats(),
       rarity: ItemRarity.common,
     );
   }
@@ -71,59 +58,39 @@ class EquipmentItem {
     );
   }
 
-  EquipmentItem copyWith({
-    String? name,
-    EquipmentSlot? slot,
-    int? level,
-    int? weight,
-    ItemStats? stats,
-    ItemRarity? rarity,
-  }) {
-    return EquipmentItem(
-      name: name ?? this.name,
-      slot: slot ?? this.slot,
-      level: level ?? this.level,
-      weight: weight ?? this.weight,
-      stats: stats ?? this.stats,
-      rarity: rarity ?? this.rarity,
-    );
+  /// 가중치 (자동 장착 비교용)
+  ///
+  /// 가중치 = 기본값 + (레벨 * 10) + (희귀도 보너스) + (스탯 합계)
+  int get itemWeight {
+    const baseValue = 10;
+    return baseValue + (level * 10) + rarity.weightBonus + stats.totalStatValue;
   }
 
-  /// JSON으로 직렬화
-  Map<String, dynamic> toJson() {
-    return {
-      'name': name,
-      'slot': slot.name,
-      'level': level,
-      'weight': weight,
-      'stats': stats.toJson(),
-      'rarity': rarity.name,
-    };
-  }
+  /// 빈 아이템 여부
+  bool get isEmpty => name.isEmpty;
 
-  /// JSON에서 역직렬화
-  factory EquipmentItem.fromJson(Map<String, dynamic> json) {
-    final slotName = json['slot'] as String? ?? 'weapon';
-    final rarityName = json['rarity'] as String? ?? 'common';
-
-    return EquipmentItem(
-      name: json['name'] as String? ?? '',
-      slot: EquipmentSlot.values.firstWhere(
-        (s) => s.name == slotName,
-        orElse: () => EquipmentSlot.weapon,
-      ),
-      level: json['level'] as int? ?? 0,
-      weight: json['weight'] as int? ?? 0,
-      stats: json['stats'] != null
-          ? ItemStats.fromJson(json['stats'] as Map<String, dynamic>)
-          : ItemStats.empty,
-      rarity: ItemRarity.values.firstWhere(
-        (r) => r.name == rarityName,
-        orElse: () => ItemRarity.common,
-      ),
-    );
-  }
+  /// 유효한 아이템 여부
+  bool get isNotEmpty => name.isNotEmpty;
 
   @override
   String toString() => name.isEmpty ? '(empty)' : name;
 }
+
+// JSON 변환 헬퍼 (세이브 파일 호환성 유지)
+EquipmentSlot _slotFromJson(String? value) {
+  return EquipmentSlot.values.firstWhere(
+    (s) => s.name == value,
+    orElse: () => EquipmentSlot.weapon,
+  );
+}
+
+String _slotToJson(EquipmentSlot slot) => slot.name;
+
+ItemRarity _rarityFromJson(String? value) {
+  return ItemRarity.values.firstWhere(
+    (r) => r.name == value,
+    orElse: () => ItemRarity.common,
+  );
+}
+
+String _rarityToJson(ItemRarity rarity) => rarity.name;