Add LytlnyblOS guide to README

[red-mutt]

What is this?

• Title: lytlnyblOS
• URL: https://github.com/red-mutt/lytlnyblOS
• Section: C/C++

Checklist

• This PR adds/fixes exactly one tutorial (or is a docs/tooling-only change).
• Free and open -- no paywall, login wall, or required newsletter signup.
• Project-based -- following it, the reader builds a complete, working artifact.
• I searched README.md for this URL and title -- it is not already listed.
• Entry uses - [Title](https://...) format, placed under the correct section (nested - [Part N](...) for multi-part series).
• No URL shorteners.
• I ran python3 scripts/check_readme.py lint locally and it exits 0.
• Relationship disclosure: I am the author.