feat(core): SkillSystemState에 GCD 및 스킬 슬롯 추가

- globalCooldownEndMs: GCD 종료 시점 추적
- globalCooldownDuration: 1500ms 상수 정의
- equippedSkills: 장착된 스킬 슬롯 (SkillSlots)
- isGlobalCooldownActive, remainingGlobalCooldown getter
- startGlobalCooldown() 메서드 추가

lib/src/core/model/game_state.dart

@@ -8,6 +8,7 @@ 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/model/skill_slots.dart';
 import 'package:asciineverdie/src/core/util/deterministic_random.dart';
 
 /// Minimal skeletal state to mirror Progress Quest structures.
@@ -193,14 +194,19 @@ enum DeathCause {
 
 /// 스킬 시스템 상태 (Phase 3)
 ///
-/// 스킬 쿨타임, 활성 버프, 게임 경과 시간 등을 관리
+/// 스킬 쿨타임, 활성 버프, 게임 경과 시간, 장착 스킬 등을 관리
 class SkillSystemState {
   const SkillSystemState({
     required this.skillStates,
     required this.activeBuffs,
     required this.elapsedMs,
+    this.equippedSkills = const SkillSlots(),
+    this.globalCooldownEndMs = 0,
   });
 
+  /// 글로벌 쿨타임 (GCD) 상수: 1500ms
+  static const int globalCooldownDuration = 1500;
+
   /// 스킬별 쿨타임 상태
   final List<SkillState> skillStates;
 
@@ -210,8 +216,26 @@ class SkillSystemState {
   /// 게임 진행 경과 시간 (밀리초, 스킬 쿨타임 계산용)
   final int elapsedMs;
 
-  factory SkillSystemState.empty() =>
-      const SkillSystemState(skillStates: [], activeBuffs: [], elapsedMs: 0);
+  /// 장착된 스킬 슬롯 (타입별 제한 있음)
+  final SkillSlots equippedSkills;
+
+  /// 글로벌 쿨타임 종료 시점 (elapsedMs 기준)
+  final int globalCooldownEndMs;
+
+  /// GCD가 활성화 중인지 확인
+  bool get isGlobalCooldownActive => elapsedMs < globalCooldownEndMs;
+
+  /// 남은 GCD 시간 (ms)
+  int get remainingGlobalCooldown =>
+      isGlobalCooldownActive ? globalCooldownEndMs - elapsedMs : 0;
+
+  factory SkillSystemState.empty() => const SkillSystemState(
+    skillStates: [],
+    activeBuffs: [],
+    elapsedMs: 0,
+    equippedSkills: SkillSlots(),
+    globalCooldownEndMs: 0,
+  );
 
   /// 특정 스킬 상태 가져오기
   SkillState? getSkillState(String skillId) {
@@ -254,13 +278,22 @@ class SkillSystemState {
     List<SkillState>? skillStates,
     List<ActiveBuff>? activeBuffs,
     int? elapsedMs,
+    SkillSlots? equippedSkills,
+    int? globalCooldownEndMs,
   }) {
     return SkillSystemState(
       skillStates: skillStates ?? this.skillStates,
       activeBuffs: activeBuffs ?? this.activeBuffs,
       elapsedMs: elapsedMs ?? this.elapsedMs,
+      equippedSkills: equippedSkills ?? this.equippedSkills,
+      globalCooldownEndMs: globalCooldownEndMs ?? this.globalCooldownEndMs,
     );
   }
+
+  /// GCD 시작 (스킬 사용 후 호출)
+  SkillSystemState startGlobalCooldown() {
+    return copyWith(globalCooldownEndMs: elapsedMs + globalCooldownDuration);
+  }
 }
 
 /// 태스크 타입 (원본 fTask.Caption 값들에 대응)
@@ -594,8 +627,11 @@ class Equipment {
   }
 
   /// 초보자 방어구 생성 헬퍼
-  static EquipmentItem _starterArmor(String name, EquipmentSlot slot,
-      {required int def}) {
+  static EquipmentItem _starterArmor(
+    String name,
+    EquipmentSlot slot, {
+    required int def,
+  }) {
     return EquipmentItem(
       name: name,
       slot: slot,