codeSplitter: Dependency doctor: 🪏 dig a future release to watch, try, and take #863

Part of #859.

🪏 Dig a future release: mark a coming version (the next major, a beta, a named tag) as "watch this and try it".

• The doctor watches the registry for it, and when it drops, tries it in the throwaway clone (tests, plus the fail chain if one is set)
• The result shows on the health page
• An optional "take it when it's green" toggle puts it on the doctor's branch automatically
• Works past max_level and the ignore list for that one package, since the owner asked for it explicitly