fix: 调整生命值特效触发阈值从99改为20

避免低生命值时频繁触发特效和提示，优化游戏体验。

assets/script/game/hero/HeroViewComp.ts

@@ -367,7 +367,7 @@ export class HeroViewComp extends CCComp {
    
     health(hp: number = 0) {
         // ✅ 仅显示特效和提示，不调用 hp_show()
-        if(hp<=99) return;
+        if(hp<=20) return;
         this.heathed();
         this.hp_tip(TooltipTypes.health, hp.toFixed(0));
         this.lastBarUpdateTime = Date.now() / 1000;