docs: add QTE explanation comment in QTESystem

engine/systems/QTESystem.ts

@@ -4,6 +4,9 @@ type QTEUpdateCallback = (remaining: number, total: number) => void
 type QTEResultCallback = (success: boolean) => void
 
 export class QTESystem {
+  // QTE (Quick Time Event / 快速反应事件):
+  // 视频播放到特定时间点时弹出按键提示，玩家在倒计时内按下指定按键，
+  // 成功/失败/超时分别导向不同剧情分支，并应用对应的 effects 效果。
   private timerId: ReturnType<typeof setInterval> | null = null
   private timeoutId: ReturnType<typeof setTimeout> | null = null
   private keyHandler: ((e: KeyboardEvent) => void) | null = null