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.
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