The option defaulted to pkgs.swarm-ui, which does not exist: this
project has no overlay - flake.nix's nixosModules.default wires package
options with mkDefault from its own package set, and swarm.controller
does exactly that. The default would have failed to evaluate on any real
deployment, not just in a test harness.
Found by the gate forcing .package: the earlier probes passed because
they only read option values that never touched it.
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