style(docs): standardize heading formats and numbering

- Enforce Level 1-3 structural numbering based on SUMMARY.md hierarchy
- Remove structural numbering from Level 4+ headings
- Eliminate single child headings by converting to bold text
- Auto-inject summary text for headings with multiple children missing intro text
- Exclude Appendix chapters from structural numbering
- Avoid modifying code block contents
- Add script to detect non-standard English usage in headers

19_cases/os/summary.md

@@ -1,4 +1,4 @@
-## 本章小结
+## 19.6 本章小结
 
 本章讲解了典型操作系统镜像的下载和使用。