The UI and the swarm-controller it fronts are one service to a reader and to a
certificate, so a separate option only ever created two spellings of one
address — and the controller's own public URL was built from one of them while
an allow-list entry named the other.
Removed rather than aliased. A config still setting it was pinning a name the
controller never used; an alias would preserve exactly that mismatch, so
`mkRemovedOptionModule` says so and fails the build.
⚠️ The option was TOTAL and `swarm.domain` is NULLABLE. That is not incidental:
`ui.domain` fell back to `swarm.invalid` so hive-network.nix's required-domain
assertion is what an operator sees, rather than a coercion error from a
consumer. Each site keeps that property its own way — swarm-ui binds a local
total (the idiom swarm-otel.nix already uses as `domainBase`), and the three
sites already gated on `deploy.swarm-ui.enable` gain `&& domain != null`, which
is stricter than a placeholder and puts no invented name into rendered config.
One behaviour change worth naming: `swarm.nix`'s certificate SAN list used to
receive the placeholder `swarm.invalid` on a null domain and now receives
nothing. `lib.optional` would otherwise have put a literal `null` in it.
Verified with `checks.module-eval` rather than `nix-instantiate --parse`: parse
proves syntax, and every failure this change could plausibly cause — a dead
binding, a renamed reference — is an evaluation error.
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