The required-domain assertion in hive-network.nix could not be reached:
`forge.<domain>` and `matrix.<domain>` are evaluated while the
assertion list is, so an unset domain threw `cannot coerce null to a
string` naming one of those options instead of printing the message
that says which option to set.
Both defaults now fall back to a name under the reserved `.invalid`
TLD, which the assertion refuses to let out the door.
747f405c6f