docs: 疑难排查表移除与已删提示重复的 --keep-frames 行

README.en.md

@@ -248,7 +248,6 @@ rules and the pre-upload checklist: [`references/cover-guide.md`](references/cov
 | Cover renders at 1× | Same trap, `deviceScaleFactor` twin | Pass to `newPage()` + `screenshot({ scale: 'device' })` |
 | Numbers render but never move | The seek suppressed `onUpdate` callbacks | Renderer fixed (`pause(t, false)`); use a transform-based number reel in scenes |
 | `No such file or directory` on Windows | Non-ASCII path — Windows ffmpeg reads UTF-8 as ANSI | Keep paths ASCII |
-| Preview finished, frame PNGs are gone | Preview mode clears the frame directory by design | Pass `--keep-frames` |
 
 **`references/lessons.md` is the most valuable file in this repository.** 34 numbered entries, each
 one a bug where "the video looked fine but was wrong" — including how it was misdiagnosed at first.

README.md

@@ -234,7 +234,6 @@ tl.fromTo('.verdict', { scale: 0.8 },          { scale: 1, duration: 0.6 },
 | 封面出成 1 倍图 | 同上，`deviceScaleFactor` 的孪生坑 | 传 `newPage()` + `screenshot({ scale: 'device' })` |
 | 数字能渲染但永远不动 | seek 抑制了 `onUpdate` 回调 | 渲染器已修（`pause(t, false)`）；帧里改用 transform 数字卷轴 |
 | Windows 上 `No such file or directory` | 非 ASCII 路径 —— Windows 的 ffmpeg 把 UTF-8 当 ANSI 读 | 路径保持 ASCII |
-| 预览渲完，帧 PNG 不见了 | 预览模式按设计会清空帧目录 | 加 `--keep-frames` |
 
 **`references/lessons.md` 是这个仓库里最值钱的文件。** 34 条编号记录，每一条都是一个
 「成片看着挺正常、其实是错的」的 bug —— 包括它一开始是怎么被误判的。从零开始 debug 之前，