Configure a router from settings.json and rebuild it on the device

nixosModules.settings builds a router from a JSON file: the router
options as they are, plus host name, time zone, SSH keys and
stateVersion; the module system validates it. lib.mkRouterSystem builds
such a system, lib.mkConfigDir the configuration directory installed to
/var/lib/router/config (a flake pinned to this router source, the
settings and an optional local.nix). router-apply builds it and switches
with commit-confirm.

The device must be able to rebuild without internet, so the packages the
rebuilt derivations need are part of the system; they were found by
diffing the derivation graphs of typical changes against the installed
system.

checks.apply installs such a router, boots it offline, checks that the
unchanged settings rebuild to the installed system, applies and
confirms a change, and rolls back an unconfirmed one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/default.nix

@@ -10,6 +10,7 @@
   rollback = import ./rollback.nix { inherit self pkgs; };
   dns = import ./dns.nix { inherit self pkgs; };
   installer = import ./installer.nix { inherit self pkgs; };
+  apply = import ./apply.nix { inherit self pkgs; };
   image = import ./image.nix { inherit self pkgs; };
   wireguard = import ./wireguard.nix { inherit self pkgs; };
   lib = import ./lib.nix { inherit pkgs; };