feat(3112): the swarm-nats container, fail-closed

A swarm has one message queue; this is the container that runs it, in
the same shape as every other swarm service. Off by default, and off
means absent.

The auth_callout block is the interim state and the final one at once.
Measured on the pinned nats-server rather than reasoned about: an empty
authorization block, and one with an empty users list, both accept an
anonymous client and answer PONG. auth_callout sets auth_required and
refuses everyone until a responder approves them, so a config whose
responder does not exist yet denies by default -- and the responder
lands beside it without any of this being swapped out.

That matters more here than it would elsewhere because the container
shares the host netns, so an unauthenticated interim state would be
reachable from every agent container on the hive.

No package option, deliberately: nixpkgs' nats module resolves
pkgs.nats-server itself and exposes none, so one here would name
something it cannot control.

No gateway vhost either: NATS speaks its own TCP protocol, nginx cannot
front it, and cross-hive reach is the wireguard mesh.

nix/host-modules/default.nix

@@ -24,6 +24,7 @@
     ./otel.nix
     ./swarm-authelia.nix
     ./swarm-ca.nix
+    ./swarm-nats.nix
     ./swarm-controller.nix
     ./swarm-snapshot-store.nix
     ./swarm-ui.nix