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;