docs: add comment explaining waitReady logic

engine/core/VideoManager.ts

@@ -141,7 +141,9 @@ export class VideoManager {
   }
 
   private waitReady(el: HTMLVideoElement): Promise<void> {
+    // readyState >= 2 (HAVE_CURRENT_DATA) 表示已有足够数据播放当前帧，无需等待
     if (el.readyState >= 2) return Promise.resolve()
+    // 否则等待 canplay 事件（浏览器判断可开始播放），同时手动触发 load 确保加载流程已启动
     return new Promise((resolve) => {
       el.addEventListener('canplay', () => resolve(), { once: true })
       el.load()