swarm-nats, matrix: the queue's and matrix-ctl's store identity paths are their own, not the hive's

glue-nats-bao-identity.nix and glue-matrix-ctl-bao-identity.nix defaulted
deploy.nats.baoClient{Cert,Key}File and deploy.matrix.ctlBaoClient{Cert,Key}File
to siblings of deploy.bao.clientCertFile, the hive's own store reader. That
pointed a store host whose reader was moved at files nobody mints, and on a
queue or homeserver host without the store that held a placed hive reader
invented a nats.pem / matrix-ctl.pem path beside it.

Both glues are now gated on the store running on this host
(deploy.bao.enable, the condition glue-bao-tls.nix mints under) and name the
directory the minting unit writes to, through nix/host-modules/lib/bao-pki-dir.nix.

Host shapes:
- Store host, default reader: unchanged. The evaluated paths stay
  /var/lib/swarm-bao-pki/nats{,-key}.pem and
  /var/lib/swarm-bao-pki/matrix-ctl{,-key}.pem, and the rendered units are
  the same.
- Store host whose hive reader was moved: now gets the leaf minted there.
- Queue or homeserver host without the store, holding a hive reader and not
  naming these options: no default any more. It still evaluates (neither
  option has an eval assertion). swarm-bao-nats-tls exits 1 naming
  deploy.nats.baoClient{Cert,Key}File when the queue's certificate is missing
  or due for renewal; swarm-bao-matrix-oidc exits 1 naming
  ctlBaoClient{Cert,Key}File, and the swarm appservice render/publish units
  and the PKI bind mount no longer render. If that host had placed nats.pem
  or matrix-ctl.pem beside its hive reader, it worked before and now needs
  both options named.
- Hosts without the store and without a hive reader, and hosts naming the
  options: unchanged.

Both refusals and option descriptions now say the default comes from
deploy.bao.enable.

Refs #4921

nix/host-modules/swarm-nats.nix

@@ -491,9 +491,12 @@ in
         `pki/issue/swarm-nats` and nothing else.
 
         No default. ./glue-nats-bao-identity.nix points it at the leaf
-        ./glue-bao-tls.nix mints, where this host mints one. On a queue host
+        ./glue-bao-tls.nix mints on a host that runs the store
+        ({option}`services.hyperhive.deploy.bao.enable`). On a queue host
         without the store, it is a file the operator copies from the store's
-        host.
+        host. Never derived from
+        {option}`services.hyperhive.deploy.bao.clientCertFile`, which is the
+        hive's own reader.
 
         A path, never a value.
       '';
@@ -1340,6 +1343,8 @@ in
               echo "cannot be requested from the store." >&2
               echo "Set services.hyperhive.deploy.nats.baoClient{Cert,Key}File" >&2
               echo "to a leaf the store's CA signed with CN=${baoDeploy.natsCommonName}." >&2
+              echo "They default to the leaf the store mints only where" >&2
+              echo "services.hyperhive.deploy.bao.enable is set on this host." >&2
               exit 1''
           else
             ""