Many upstream plugins publish no tags at all, so a tag-only installer cannot
manage them.
- Add a branch spec field, mutually exclusive with tag and validated the same way
- Build archive/refs/heads/<branch> URLs for branch specs
- Refetch a branch only on :ArtiPackUpdate or a forced sync, never from cache
- Record the branch, archive digest and fetch time in the lockfile
- Report a branch plugin as tracked and fetched, never as up to date
