refactor(swarm-queue-client): share the hive-status bucket's name and shape

The bucket has two ends in two crates: a hive writes its own key, the
controller reads every key. `swarm-controller` declared the name as a
private const with a doc comment arguing that "reader and writer must
name the same bucket" — an argument the writer, in another crate, could
not obey.

The name is the mild half. Both ends do get-or-create, because either may
come up first on a fresh swarm and neither can assume the other has run.
Two `Config`s that drift means whichever end created the bucket wins and
the other's `get_key_value` succeeds against a bucket it did not ask for:
no error, no log, just a retention policy nobody chose. Sharing the
constructor gives that race one outcome.

Behind a default-off `kv` feature, so the crate's other consumer — the
auth-callout responder, which speaks the connect and nothing else — still
pulls neither `jetstream` nor `kv`. That was the actual reason the
feature was excluded when this crate was extracted; the flag preserves
it. The surface is deliberately narrow: one bucket's name and creation
config, not a general KV facade.

swarm-queue-client/README.md

@@ -50,6 +50,23 @@ effect actually did.
 
 ## What this crate does not do
 
-It ends at a connected client. No `jetstream`/`kv` feature is enabled here —
-what a consumer does with the connection is its own business, and its
-`Cargo.toml` is where that requirement should be visible.
+It ends at a connected client. `jetstream`/`kv` are **off by default** — what a
+consumer does with the connection is its own business, and its `Cargo.toml` is
+where that requirement should be visible. The auth-callout responder speaks the
+connect and nothing else, and pays for nothing else.
+
+## The one exception: the `kv` feature
+
+`kv` adds `status`, which holds the name and the creation config of the
+`hive-status` bucket — nothing more.
+
+It is here because that bucket has **two ends in two crates**: a hive writes its
+own key, the controller reads every key. The name being a repeated literal is
+the mild half of the problem; the sharp half is that either end may arrive first
+on a fresh swarm, so both create the bucket if it is missing. Two `Config`s that
+drift means whichever end created it wins and the other opens a bucket it did
+not ask for — no error, no log, just a retention policy nobody chose.
+
+An agreement between two crates has to live in one of them, and neither end of
+this bucket is senior to the other. Behind a default-off feature, the consumer
+that needs none of it still pays nothing.