feat(balance): CHA(매력) 스탯 활성화 — 상점 할인 + 드롭률 보정

- shop_service: CHA 기반 구매 할인 (CHA-10)×1%, 최대 15%
- item_service: CHA 기반 희귀 아이템 드롭률 보정 (CHA-10)×0.5%
- 호출부(game_mutations, market_service, resurrection_service)에 CHA 전달
- 기존 데드 스탯이었던 CHA가 게임 로직에 반영됨

lib/src/core/engine/game_mutations.dart

@@ -26,6 +26,7 @@ class GameMutations {
       name: name,
       slot: slot,
       level: level,
+      cha: state.stats.cha,
     );
 
     final updatedEquip = state.equipment

lib/src/core/engine/item_service.dart

@@ -60,23 +60,34 @@ class ItemService {
   // 희귀도 결정
   // ============================================================================
 
-  /// 희귀도 결정 (고정 확률)
+  /// 희귀도 결정 (고정 확률 + CHA 보정)
   ///
-  /// 확률 분포:
+  /// 기본 확률 분포:
   /// - Common: 34%
   /// - Uncommon: 40%
   /// - Rare: 20%
   /// - Epic: 5%
   /// - Legendary: 1%
-  ItemRarity determineRarity(int level) {
+  ///
+  /// CHA 보정(charisma bonus): (CHA - 10) * 0.5% 추가 희귀 확률.
+  /// 보정값만큼 Common 확률이 줄고, Rare 이상 확률이 증가.
+  ItemRarity determineRarity(int level, {int cha = 0}) {
     final roll = rng.nextInt(100);
 
-    // Legendary: 0-0 (1%)
-    if (roll < 1) return ItemRarity.legendary;
-    // Epic: 1-5 (5%)
-    if (roll < 6) return ItemRarity.epic;
-    // Rare: 6-25 (20%)
-    if (roll < 26) return ItemRarity.rare;
+    // CHA 보정: (CHA - 10) * 0.5, 0~10 범위
+    final chaBonus = ((cha - 10) * 0.5).clamp(0.0, 10.0);
+
+    // 보정된 임계값 (chaBonus만큼 희귀 쪽으로 이동)
+    final legendaryThreshold = 1.0 + chaBonus * 0.1; // 최대 2%
+    final epicThreshold = 6.0 + chaBonus * 0.3; // 최대 9%
+    final rareThreshold = 26.0 + chaBonus * 0.6; // 최대 32%
+
+    // Legendary
+    if (roll < legendaryThreshold) return ItemRarity.legendary;
+    // Epic
+    if (roll < epicThreshold) return ItemRarity.epic;
+    // Rare
+    if (roll < rareThreshold) return ItemRarity.rare;
     // Uncommon: 26-65 (40%)
     if (roll < 66) return ItemRarity.uncommon;
     // Common: 66-99 (34%)
@@ -331,8 +342,9 @@ class ItemService {
     required EquipmentSlot slot,
     required int level,
     ItemRarity? rarity,
+    int cha = 0,
   }) {
-    final itemRarity = rarity ?? determineRarity(level);
+    final itemRarity = rarity ?? determineRarity(level, cha: cha);
     final stats = generateItemStats(
       level: level,
       rarity: itemRarity,

lib/src/core/engine/market_service.dart

@@ -56,7 +56,7 @@ class MarketService {
         slot: slot,
         targetRarity: ItemRarity.common,
       );
-      final price = shopService.calculateBuyPrice(item);
+      final price = shopService.calculateBuyPrice(item, cha: state.stats.cha);
 
       if (nextState.inventory.gold >= price) {
         nextState = nextState.copyWith(

lib/src/core/engine/resurrection_service.dart

@@ -105,6 +105,7 @@ class ResurrectionService {
       playerLevel: state.traits.level,
       currentGold: state.inventory.gold,
       currentEquipment: state.equipment,
+      cha: state.stats.cha,
     );
 
     // 장비 적용

lib/src/core/engine/shop_service.dart

@@ -18,10 +18,21 @@ class ShopService {
 
   /// 장비 구매 가격 계산
   ///
-  /// 가격 = 아이템 레벨 * 50 * 희귀도 배율
-  int calculateBuyPrice(EquipmentItem item) {
+  /// 가격 = 아이템 레벨 * 50 * 희귀도 배율 * (1 - CHA 할인율)
+  /// CHA 할인율(charisma discount): (CHA - 10) * 1%, 최대 15%, 최소 0%
+  int calculateBuyPrice(EquipmentItem item, {int cha = 0}) {
     if (item.isEmpty) return 0;
-    return (item.level * 50 * _getRarityPriceMultiplier(item.rarity)).round();
+    final basePrice = (item.level * 50 * _getRarityPriceMultiplier(item.rarity))
+        .round();
+    final discount = chaDiscount(cha);
+    return (basePrice * (1.0 - discount)).round();
+  }
+
+  /// CHA 기반 할인율 계산
+  ///
+  /// (CHA - 10) * 0.01, 0~0.15 범위로 클램프
+  static double chaDiscount(int cha) {
+    return ((cha - 10) * 0.01).clamp(0.0, 0.15);
   }
 
   /// 장비 판매 가격 계산
@@ -211,6 +222,7 @@ class ShopService {
     required int playerLevel,
     required int currentGold,
     required Equipment currentEquipment,
+    int cha = 0,
   }) {
     var remainingGold = currentGold;
     final purchasedItems = <EquipmentItem>[];
@@ -230,7 +242,7 @@ class ShopService {
         targetRarity: ItemRarity.common, // 부활 시 Common만 구매
       );
 
-      final price = calculateBuyPrice(shopItem);
+      final price = calculateBuyPrice(shopItem, cha: cha);
       if (price <= remainingGold) {
         remainingGold -= price;
         purchasedItems.add(shopItem);
@@ -254,6 +266,7 @@ class ShopService {
     required int currentGold,
     required EquipmentSlot slot,
     ItemRarity? preferredRarity,
+    int cha = 0,
   }) {
     final item = generateShopItem(
       playerLevel: playerLevel,
@@ -261,7 +274,7 @@ class ShopService {
       targetRarity: preferredRarity,
     );
 
-    final price = calculateBuyPrice(item);
+    final price = calculateBuyPrice(item, cha: cha);
     if (price > currentGold) return null;
 
     return PurchaseResult(