refactor: 调整卡牌等级逻辑，新增基础池等级字段

1. 新增base_pool_lv字段存储英雄初始池等级
2. 替换多处直接使用pool_lv的逻辑，优先使用base_pool_lv
3. 移除预制体中废弃的pool_lv_node节点

assets/script/game/hero/Hero.ts

@@ -111,6 +111,7 @@ export class Hero extends ecs.Entity {
         model.hero_name = hero.name;
         model.lv = hero_lv;
         model.pool_lv = pool_lv;
+        model.base_pool_lv = hero.pool_lv ?? 1;
         model.type = hero.type;
         model.fac = FacSet.HERO;
         model.dis = hero.dis ?? 720;

assets/script/game/hero/HeroAttrsComp.ts

@@ -20,6 +20,7 @@ export class HeroAttrsComp extends ecs.Comp {
     hero_name: string = "hero";
     lv: number = 1;
     pool_lv: number = 1;
+    base_pool_lv: number = 1;
     type: number = 0;           // 0近战 1远程 2辅助
     fac: number = 0;            // 0:hero 1:monster
     // ==================== 基础属性（有初始值） ====================