refactor(model): game_state.dart 분할 (SRP 개선)

- Stats, Traits, Inventory, Equipment 등 11개 파일로 분리
- 단일 책임 원칙 적용으로 유지보수성 향상

lib/src/core/model/stats.dart

@@ -0,0 +1,75 @@
+/// 캐릭터 스탯 (Stats)
+///
+/// 6대 능력치(STR, CON, DEX, INT, WIS, CHA)와 HP/MP를 관리
+class Stats {
+  const Stats({
+    required this.str,
+    required this.con,
+    required this.dex,
+    required this.intelligence,
+    required this.wis,
+    required this.cha,
+    required this.hpMax,
+    required this.mpMax,
+    this.hpCurrent,
+    this.mpCurrent,
+  });
+
+  final int str;
+  final int con;
+  final int dex;
+  final int intelligence;
+  final int wis;
+  final int cha;
+  final int hpMax;
+  final int mpMax;
+
+  /// 현재 HP (null이면 hpMax와 동일)
+  final int? hpCurrent;
+
+  /// 현재 MP (null이면 mpMax와 동일)
+  final int? mpCurrent;
+
+  /// 실제 현재 HP 값
+  int get hp => hpCurrent ?? hpMax;
+
+  /// 실제 현재 MP 값
+  int get mp => mpCurrent ?? mpMax;
+
+  factory Stats.empty() => const Stats(
+    str: 0,
+    con: 0,
+    dex: 0,
+    intelligence: 0,
+    wis: 0,
+    cha: 0,
+    hpMax: 0,
+    mpMax: 0,
+  );
+
+  Stats copyWith({
+    int? str,
+    int? con,
+    int? dex,
+    int? intelligence,
+    int? wis,
+    int? cha,
+    int? hpMax,
+    int? mpMax,
+    int? hpCurrent,
+    int? mpCurrent,
+  }) {
+    return Stats(
+      str: str ?? this.str,
+      con: con ?? this.con,
+      dex: dex ?? this.dex,
+      intelligence: intelligence ?? this.intelligence,
+      wis: wis ?? this.wis,
+      cha: cha ?? this.cha,
+      hpMax: hpMax ?? this.hpMax,
+      mpMax: mpMax ?? this.mpMax,
+      hpCurrent: hpCurrent ?? this.hpCurrent,
+      mpCurrent: mpCurrent ?? this.mpCurrent,
+    );
+  }
+}