refactor(mission): 移除英雄二合一合成逻辑，改为允许同英雄同场

1.  删除Hero类中的mergeToBirthAndDestroy方法
2.  移除MissionHeroComp中的合成相关校验与处理逻辑
3.  简化英雄召唤流程，不再限制同uuid英雄重复召唤
4.  更新注释文档，调整相关导入依赖

assets/script/game/hero/Hero.ts

@@ -268,36 +268,6 @@ export class Hero extends ecs.Entity {
             .start();
     }
 
-    mergeToBirthAndDestroy(birthPos: Vec3, onDone?: () => void) {
-        const view = this.get(HeroViewComp);
-        const node = view?.node;
-        if (!node || !node.isValid) {
-            this.destroy();
-            if (onDone) onDone();
-            return;
-        }
-        const collider = node.getComponent(BoxCollider2D);
-        if (collider) {
-            collider.enabled = false;
-        }
-        const body = node.getComponent(RigidBody2D);
-        if (body) {
-            body.enabled = false;
-        }
-        const currentPos = node.getPosition();
-        const targetPos = v3(birthPos.x, birthPos.y, 0);
-        const moveDistance = Vec3.distance(currentPos, targetPos);
-        const moveDuration = Math.max(0.12, Math.min(0.3, moveDistance / 1200));
-        Tween.stopAllByTarget(node);
-        tween(node)
-            .to(moveDuration, { position: targetPos })
-            .call(() => {
-                this.destroy();
-                if (onDone) onDone();
-            })
-            .start();
-    }
-
     /** 重置入口：复用 destroy 的释放流程 */
     reset() {
         super.destroy();