argus on PR #754 v2 review:
> `subdomain = ""` edge case: when `cfg.subdomain = ""`, the
> `localHostsEntry` appends ".${domain}" (invalid hostname; bare
> domain is already covered) and the virtualHosts key becomes
> ".${domain}" (nginx treats this as a wildcard catch-all, not a
> bare-domain server block). docs call this "advanced: collides
> with dashboard server block" — the actual nginx behavior is
> more surprising than that.
Fix: reject `""` at assertion time rather than ship the surprising
behaviour. Bare-domain landing is what the dashboard already
serves; there's no use case for `""` that null doesn't already
cover. Updated option description + dropped the now-dead branch
from the `subdomain` let-binding.
Verified: `services.hyperhive.forge.subdomain = ""` triggers the
new assertion at toplevel build with a clear message pointing at
`null` as the right opt-out. Default + `null` paths still build
clean.
de67002c94