Neither has a fallback worth having: a guessed swarm domain is a wrong
hostname that evaluates cleanly and deploys, which is worse than an
eval failure naming the one line an operator has to write. Upgrading
past this sets both, once.
Requiring them also makes the hive domain fully derived rather than
merely derivable — `<hiveName>.<swarm.domain>` now always resolves, so
an operator writes the swarm's address and this hive's label and never
writes the hive domain at all.
`hiveName` stops being display-only in the process: it is the leftmost
label of the domain the hive is addressed by, which the option text and
docs now say.
Each of the three required options asserts separately, so a missing one
names itself. A missing `swarm.domain` legitimately fails two of them —
its own, and `domain`, which can no longer derive — and nix reports all
failing assertions together, so the operator sees the whole set rather
than one at a time.
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.
Every hive in a swarm occupies its own sub-domain of the swarm's, so
the hive domain is derivable rather than something each hive restates.
`services.hyperhive.swarm.domain` is new and nullable; the hive's own
`domain` keeps its existing required-ness and its existing assertion,
and gains a default of `<hiveName>.<swarm.domain>`.
Deliberately a default and not a rename: an alias would reinterpret the
domains hives have already deployed, while a default only fills in the
ones that never set one. Same reason there is no new assertion — the
existing message just names the derivation, so this adds a way to stop
failing rather than a way to fail.
