refactor(swarm-queue-client): typed errors for the bucket and the guard

Finishes the anyhow removal for the parts this branch adds: the status
bucket's open-or-create and the connected-client precondition. Two
variants, one of them behind the `kv` feature because the error type it
wraps does not exist without it — the error enum respects the same gate
the module does.

NotConnected is deliberately distinct from Connect: one is a connect that
was attempted and refused, the other is a request made before any
connection exists. The first is a deployment problem and the second is a
caller-ordering one, which is the whole reason a caller wants an enum
rather than a string.

The controller's `store` now returns the queue client's error rather than
an anyhow one: `OnceCell::get_or_try_init` takes its error type from the
closure, so widening there would mean converting inside the closure for
no gain. `view` `?`s it and anyhow converts at that boundary — the
library keeps a typed error, the binary keeps anyhow, and no call site
pays for the split.

swarm-queue-client/src/status.rs

@@ -19,7 +19,7 @@
 //! responder, still pulls neither `jetstream` nor `kv`: it speaks the
 //! connect and nothing else.
 
-use anyhow::{Context, Result};
+use crate::Error;
 
 /// The KV bucket hives publish their status snapshots into, one key per
 /// hive keyed by `hiveName`.
@@ -41,7 +41,7 @@ pub const BUCKET: &str = "hive-status";
 /// into a permanent, silent absence of data.
 pub async fn open_or_create(
     client: &async_nats::Client,
-) -> Result<async_nats::jetstream::kv::Store> {
+) -> Result<async_nats::jetstream::kv::Store, Error> {
     let js = async_nats::jetstream::new(client.clone());
     match js.get_key_value(BUCKET).await {
         Ok(store) => Ok(store),
@@ -58,7 +58,10 @@ pub async fn open_or_create(
                 ..Default::default()
             })
             .await
-            .with_context(|| format!("creating the {BUCKET} bucket"))
+            .map_err(|source| Error::CreateBucket {
+                bucket: BUCKET,
+                source,
+            })
         }
     }
 }