swarm-nats: give the queue a name, a bao-issued leaf, and require TLS

The queue listened in plaintext on 4222, reached by bridge IP or loopback,
and nothing in-tree opened it to another hive. It now has a name, serves a
certificate for that name alone, and refuses clients that do not speak TLS.

- `swarm.nats.domain`, default `nats.<swarm.domain>`, a sibling name like
  `swarm.bao.domain`. The queue host answers it via `gateway.localNames`;
  every other hive resolves it through the operator's DNS, as for bao.
- `pki/roles/swarm-nats` allows that one name (bare domain, no subdomains,
  IPs or localhost, server flag). A `swarm-nats` cert-auth role and policy
  may only `update` `pki/issue/swarm-nats`, written by
  `swarm-bao-nats-tls-policy`. The login leaf is minted by glue-bao-tls and
  paired by glue-nats-bao-identity. `deploy.bao.natsCommonName` is reserved
  as a hive name.
- `swarm-bao-nats-tls` issues the leaf into a directory bound read-only into
  the container, restarts nats when it rotates, and re-runs daily.
  It joins glue-bao-readers-policy-order, so it is ordered after its policy
  unit (`after` and `wants`, never `requires`) where the store is on the
  same host. The policy unit joins the store's journald list.
- nats gets `tls {}`, with the key via `LoadCredential`, and no
  `allow_non_tls`. `validateConfig` is now off in every mode, because the
  build-time check loads a leaf that only exists at runtime.
- 4222 is also open on `wg-hive` when the host is on the mesh, never
  host-wide.
- `statusPublish.natsUrl`, `queue.agentNatsUrl`, the controller's URL under
  `singleHostSwarm`, and the auth responder all dial
  `tls://<swarm.nats.domain>:<port>`. swarm-queue-client hands its CA file
  to the NATS connection too, so hive-c0re and the controller trust the
  leaf's root.
- docs/swarm/README.md: the queue URL and the one DNS record a multi-host
  swarm needs.

module-eval-nats-tls pins the role, the policy, the served leaf, the
firewall, the ordering, and a scan of every `*_NATS_URL` and the
responder's URL across the host and its containers.

Closes #4626

nix/host-modules/swarm-otel.nix

@@ -475,10 +475,8 @@ in
         collector is a plain producer against this name exactly like every
         other client of a swarm service, resolved locally by dnsmasq on a
         co-located host and over the real network otherwise. There is no
-        separate loopback-vs-remote knob to get wrong: `swarm-nats` is the
-        deliberate exception to this pattern (its cross-hive reach is the
-        wireguard mesh, not the gateway), everything else in this swarm
-        addresses its siblings by name.
+        separate loopback-vs-remote knob to get wrong: everything in this
+        swarm addresses its siblings by name.
       '';
     };