Stability tags

Each docs page can declare a stability tag in frontmatter.

stability: stable        # stable | working | experimental

Tag	Meaning
stable	Current direction. Build against it unless a newer decision log entry says otherwise.
working	Current draft. Good enough to implement behind review, but expect iteration.
experimental	Research or design exploration. Do not treat it as an implementation contract.

Removed roadmap material is not represented with an archived tag. Git history is the historical record.