swarm-nats: manual callout needs all four keys, not two

The two callout assertions guarded the halves the server verifies with.
The responder needs the other halves, and nothing related them: a config
supplying only the public keys satisfies both, renders a syntactically
valid auth_callout block, and defines no responder unit.

Callout with no responder is the fail-closed state, so that queue refuses
every client — and a NATS denial arrives as a timeout, so the symptom is
every consumer hanging with nothing logged anywhere.

The build-time config check does run in this case and passes, because the
config is valid; what is missing is a unit, and the absence of a unit is
not an event.

nix/host-modules/swarm-nats.nix

@@ -448,6 +448,39 @@ in
           and no key authenticates anyone who sends that name.
         '';
       }
+      {
+        # The two above guard the halves the SERVER needs; the responder
+        # needs the other halves, and nothing related them. Satisfying
+        # only the public ones renders a valid `auth_callout` block and
+        # defines no responder — and callout with no responder is the
+        # fail-closed state, so the queue refuses everyone.
+        #
+        # Nothing downstream catches it. `nats-server -t` runs in exactly
+        # this case and passes, because the config IS valid; the missing
+        # piece is a unit, and the absence of a unit is not an event. The
+        # symptom is every client timing out, since a NATS denial reaches
+        # the client as a timeout rather than an error.
+        # The binding itself, not a copy of its formula: what is being
+        # asserted IS "the responder is configured", and two copies of one
+        # boolean is two places for a future edit to land in only one.
+        assertion = responderConfigured;
+        message = ''
+          services.hyperhive.swarm.nats has callout public keys but no
+          seed files:
+
+            nats.calloutUserSeedFile   = "${cfg.calloutUserSeedFile}"
+            nats.calloutIssuerSeedFile = "${cfg.calloutIssuerSeedFile}"
+
+          Each seed is the private half of the public key already set
+          here — the server verifies with the public half, the responder
+          signs with the private one. Configuring one side alone leaves
+          this queue with an auth-callout nobody answers, which refuses
+          every client rather than degrading.
+
+          Set both seed paths, or set nats.autoGenerateCallout = true to
+          have this host mint all four.
+        '';
+      }
       {
         assertion = autheliaUrl != null;
         message = ''