swarm: move the queue's responder package to deploy

`swarm.nats.authPackage` is the callout responder's build — a host
decision like every other package in this slice — so it joins the
`deploy.nats` block that already holds `enable`, the seed paths and the
auto-mint toggle. Its one reader is the `ExecStart` in this file.

Three comments moved with it, and none of them survived the move
unedited:

- The "deliberately NO `package` option" note now says *anywhere* —
  it argued against a `package` under `swarm.nats`, and after the move
  a reader's next question is why there isn't one under `deploy.nats`
  either, where every other service's build now lives.
- The description's "see the note above" stopped resolving: the note
  is ~80 lines up and in a different option block, so it names
  `options.services.hyperhive.swarm.nats` explicitly.
- The split-rationale comment listed "the responder package" among
  what the queue IS to every hive. It's what the host running it
  decides, so it moved to that half of the sentence.

`flake.nix` writes this option (`lib.mkDefault`), so the writer is
repointed too — three of those left, all in the same block.

The eval fixture gains the old path rather than a new case: the shim
is what a dropped rename would break, and `natsOldPath` already fails
the eval if any single entry goes missing.

nix/host-modules/deploy.nix

@@ -369,6 +369,10 @@ in
       [ "services" "hyperhive" "swarm" "ui" "package" ]
       [ "services" "hyperhive" "deploy" "swarm-ui" "package" ]
     )
+    (lib.mkRenamedOptionModule
+      [ "services" "hyperhive" "swarm" "nats" "authPackage" ]
+      [ "services" "hyperhive" "deploy" "nats" "authPackage" ]
+    )
   ];
 
   # ⚠️ `deploy.forgejo` is declared in ./hive-ci.nix, not here, and it is the