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.

docs/swarm/secrets.md

@@ -101,6 +101,13 @@ eval, naming the option it wants. That is deliberate: a queue that started
 without them would accept `CONNECT {"user":"auth"}` from anyone sharing the
 host's network namespace, and nothing would look wrong until somebody connected.
 
+**All four or none** — the seed paths are required too, not just the public
+keys. They are two halves of the same pair: the server verifies with the public
+half, the responder signs with the private one. Supplying only the public keys
+used to pass eval and leave the queue with an auth-callout nobody answers, which
+refuses every client rather than degrading — and a refusal reaches the client as
+a timeout, so the symptom is every consumer hanging with nothing logged.
+
 One consequence of the generated path worth knowing before you debug it: with
 `autoGenerateCallout` set, the queue's config is assembled at boot rather than at
 build time, so a malformed one surfaces when the container starts instead of