fix shutdown to send deregister heartbeat before pre_stop delay

Go's StopHeartbeat() closes stopChan immediately on interrupt, causing
the heartbeat goroutine to send the deregister heartbeat right away,
before the preStopSeconds delay. Rust was only setting is_stopping=true
without waking the heartbeat loop, so the deregister was delayed until
after the pre_stop sleep. Now we call volume_state_notify.notify_one()
to wake the heartbeat immediately.

seaweed-volume/src/main.rs

@@ -486,6 +486,10 @@ async fn run(
             info!("Received shutdown signal...");
         }
         *state_shutdown.is_stopping.write().unwrap() = true;
+        // Wake heartbeat loop immediately so it sends deregister heartbeat
+        // before the pre_stop delay (matches Go: StopHeartbeat() closes stopChan
+        // before sleeping preStopSeconds)
+        state_shutdown.volume_state_notify.notify_one();
 
         // Graceful drain: wait pre_stop_seconds before shutting down servers
         let pre_stop = state_shutdown.pre_stop_seconds;