feat(game): add elite monster kill statistics and related fields

1. add is_elite field to HeroAttrsComp for elite monster mark
2. set elite monster flag when spawning mission monsters
3. implement monster kill counting logic with priority: Boss > elite > normal monsters
4. add kill count statistics for different monster types for settlement calculation

assets/script/game/hero/HeroAttrsComp.ts

@@ -125,6 +125,8 @@ export class HeroAttrsComp extends ecs.Comp {
     is_atking: boolean = false;  // 是否正在攻击
     is_stop: boolean = false;    // 是否正在停止
     is_boss: boolean = false;
+    /** 是否精英怪（刷怪时由 MissionMonComp 写入，驱动精英击杀统计） */
+    is_elite: boolean = false;
     is_big_boss: boolean = false;
     is_master: boolean = false;
     is_friend: boolean = false;