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-controller/src/status.rs

@@ -155,7 +155,16 @@ impl StatusReader {
     ///
     /// Whichever side arrives first creates it, and both sides ask for the
     /// same shape, so this is a race with one outcome.
-    async fn store(&self) -> Result<&async_nats::jetstream::kv::Store> {
+    ///
+    /// Returns the queue client's own error rather than an `anyhow::Error`:
+    /// `OnceCell::get_or_try_init` takes its error type from the closure,
+    /// so widening here would mean converting *inside* the closure for no
+    /// gain. `view` below `?`s it and anyhow converts there — which is the
+    /// whole point of the library keeping a typed error while the binary
+    /// keeps anyhow.
+    async fn store(
+        &self,
+    ) -> std::result::Result<&async_nats::jetstream::kv::Store, swarm_queue_client::Error> {
         self.store
             .get_or_try_init(|| swarm_queue_client::status::open_or_create(&self.client))
             .await

swarm-queue-client/src/lib.rs

@@ -84,6 +84,21 @@ pub enum Error {
         #[source]
         source: async_nats::ConnectError,
     },
+
+    /// Distinct from [`Error::Connect`] on purpose: that one is a connect
+    /// that was attempted and refused, this one is a request made before
+    /// any connection exists. The second is a caller-ordering problem and
+    /// the first is a deployment one.
+    #[error("not connected to the swarm queue (client state: {0:?})")]
+    NotConnected(async_nats::connection::State),
+
+    #[cfg(feature = "kv")]
+    #[error("creating the {bucket} bucket")]
+    CreateBucket {
+        bucket: &'static str,
+        #[source]
+        source: async_nats::jetstream::context::CreateKeyValueError,
+    },
 }
 
 /// Render an error and its source chain on one line.
@@ -242,10 +257,10 @@ async fn mint_token(http: &reqwest::Client, cfg: &QueueConfig) -> Result<String,
 ///
 /// Naming the state is also the better error: "not connected" is actionable,
 /// a timeout is not.
-pub fn ensure_connected(client: &async_nats::Client) -> Result<()> {
+pub fn ensure_connected(client: &async_nats::Client) -> Result<(), Error> {
     let state = client.connection_state();
     if state != async_nats::connection::State::Connected {
-        bail!("not connected to the swarm queue (client state: {state:?})");
+        return Err(Error::NotConnected(state));
     }
     Ok(())
 }

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,
+            })
         }
     }
 }