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.g.dart

@@ -0,0 +1,27 @@
+// GENERATED CODE - DO NOT MODIFY BY HAND
+
+part of 'equipment_item.dart';
+
+// **************************************************************************
+// JsonSerializableGenerator
+// **************************************************************************
+
+_$EquipmentItemImpl _$$EquipmentItemImplFromJson(Map<String, dynamic> json) =>
+    _$EquipmentItemImpl(
+      name: json['name'] as String,
+      slot: _slotFromJson(json['slot'] as String?),
+      level: (json['level'] as num).toInt(),
+      weight: (json['weight'] as num).toInt(),
+      stats: ItemStats.fromJson(json['stats'] as Map<String, dynamic>),
+      rarity: _rarityFromJson(json['rarity'] as String?),
+    );
+
+Map<String, dynamic> _$$EquipmentItemImplToJson(_$EquipmentItemImpl instance) =>
+    <String, dynamic>{
+      'name': instance.name,
+      'slot': _slotToJson(instance.slot),
+      'level': instance.level,
+      'weight': instance.weight,
+      'stats': instance.stats,
+      'rarity': _rarityToJson(instance.rarity),
+    };