swarm-nats-auth: grant each hive its own notices publish subject

`hive_subjects` granted `$JS.API.STREAM.INFO`/`CREATE` on `hive-notices`
but never the per-hive subject a publish actually targets, so every
`notices::publish` was refused with a permissions violation naming
`hive-notices.<hive>`.

The gap survived because `notices_subjects`'s own doc comment asserted
`extra_hive_subjects` covered it "since that one *is* per-hive". Nothing
in `nix/` has ever passed `--hive-publish-subject` (`git grep` → 0 hits;
control: `--reader-client` is passed at `nix/host-modules/swarm-nats.nix:711`),
so the publish had been refused for as long as the feature existed.

Subject comes from `swarm_queue_client::notices::subject`, the same
constant the publisher formats from, rather than a literal here.

Refs #3859

swarm-nats-auth/src/policy.rs

@@ -189,8 +189,8 @@ impl Policy {
     /// no such per-hive split at the `STREAM.INFO`/`STREAM.CREATE` layer
     /// — the stream itself, not a slice of it, is what every hive's
     /// `open_or_create` needs to reach before it can publish to its own
-    /// `hive-notices.<hive>` subject (which `extra_hive_subjects` already
+    /// `hive-notices.<hive>` subject, which [`Self::hive_subjects`] grants
-    /// covers, since that one *is* per-hive). Granting these two subjects
+    /// beside these from the same crate constant. Granting these two subjects
     /// to every hive is therefore correct, not a widening: it is
     /// `CREATE`/`INFO` on one named stream, the same shape already
     /// measured safe for the hive-status bucket in [`Self::create`] —
@@ -233,6 +233,11 @@ impl Policy {
             format!("$KV.{}.{hive}", self.bucket),
         ]);
         subjects.extend(Self::notices_subjects());
+        // The hive's own slice of the stream the subjects above let it open.
+        // Named from the defining crate so a rename cannot leave the grant
+        // pointing at a subject nobody publishes to. Not `extra_hive_subjects`:
+        // that is for streams this crate does not know about.
+        subjects.push(swarm_queue_client::notices::subject(hive));
         subjects.extend(
             self.extra_hive_subjects
                 .iter()
@@ -544,6 +549,27 @@ mod tests {
         );
     }
 
+    #[test]
+    fn a_hive_may_publish_to_its_own_notices_subject() {
+        // Opening the stream and writing into it are separate grants.
+        let p = policy()
+            .permissions("hive-alpha")
+            .expect("a hive is admitted");
+        assert!(p.publish.contains(&"hive-notices.alpha".to_owned()));
+    }
+
+    #[test]
+    fn a_hive_may_not_publish_to_another_hives_notices_subject() {
+        // The stream-level subjects are identical for every hive; this one
+        // must not be, or reaching the stream would carry writing to every
+        // other hive's slice of it.
+        let p = policy()
+            .permissions("hive-alpha")
+            .expect("a hive is admitted");
+        assert!(!p.publish.contains(&"hive-notices.beta".to_owned()));
+        assert!(!p.publish.iter().any(|s| s == "hive-notices.>"));
+    }
+
     #[test]
     fn the_notices_stream_grant_is_identical_across_hives() {
         // Unlike `$KV.<bucket>.<hive>` or an `extra_hive_subjects`