wip(#4006): per-hive wanted-state buckets — client crate only

CHECKPOINT, NOT A PROPOSAL. mara paused the plan ("i dont quite understand
the plan") before any behaviour-changing edit; this commit exists so the work
survives a container stop, not because it is ready.

wanted.rs: BUCKET -> BUCKET_PREFIX + bucket(hive) producing hive-wanted-<hive>;
open_or_create/open_read_only take the hive. lib.rs: Error::CreateBucket.bucket
becomes String, since a per-hive name is built at runtime.

Nothing else is touched, so no caller compiles against the new signatures yet
and no deployed behaviour changes. Remaining, if she approves: the two other
CreateBucket sites, swarm-controller's OnceCell (one store -> N), hive-c0re's
pull, policy.rs per-hive streams + the reader roster, swarm-nats.nix.

swarm-queue-client/src/lib.rs

@@ -111,7 +111,9 @@ pub enum Error {
     #[cfg(feature = "kv")]
     #[error("creating the {bucket} bucket")]
     CreateBucket {
-        bucket: &'static str,
+        // `String` rather than `&'static str`: a per-hive bucket name is built
+        // at runtime (`wanted::bucket`), so the error has to own it.
+        bucket: String,
         #[source]
         source: async_nats::jetstream::context::CreateKeyValueError,
     },