fix: 修复英雄升2级时费用未指数增长的问题

升级到2级时，费用应乘以 MERGE_NEED 的 (2-1) 次方，以符合指数增长的设计。

assets/script/game/common/config/CardSet.ts

@@ -1,5 +1,6 @@
 import * as exp from "constants"
 import { HeroInfo, HType } from "./heroSet"
+import { FightSet } from "./GameSet"
 
 /** 卡牌大类定义 */
 export enum CardType {
@@ -174,7 +175,8 @@ const applyHeroLv2Probability = (cards: CardConfig[], currentPoolLv: number): Ca
         if (card.type === CardType.Hero && card.hero_lv === 1) {
             const prob = HERO_LV2_INIT_PROB + HERO_LV2_PROB_INC_PER_LV * (currentPoolLv - card.pool_lv)
             if (Math.random() < prob) {
-                return { ...card, hero_lv: 2 }
+                // 升级到2级时，花费呈指数增长 (1级cost * MERGE_NEED^(2-1))
+                return { ...card, hero_lv: 2, cost: card.cost * Math.pow(FightSet.MERGE_NEED, 1) }
             }
         }
         return card