fix heartbeat deadlock: send initial messages before awaiting stream

The bidirectional gRPC stream had a deadlock: send_heartbeat().await
waits for response headers from the server, but the Go master won't
send response headers until it receives the first heartbeat message.
Pre-populating the channel before creating the stream resolves this.

Tested end-to-end: Rust volume server successfully registers with Go
master, receives volume assignments, and handles uploads/downloads.

seaweed-volume/src/server/heartbeat.rs

@@ -98,15 +98,16 @@ async fn do_heartbeat(
     let mut client = SeaweedClient::new(channel);
 
     let (tx, rx) = tokio::sync::mpsc::channel::<master_pb::Heartbeat>(32);
-    let stream = tokio_stream::wrappers::ReceiverStream::new(rx);
-    let mut response_stream = client.send_heartbeat(stream).await?.into_inner();
 
-    // Send initial volume heartbeat
+    // Send initial heartbeats BEFORE calling send_heartbeat to avoid deadlock:
+    // the server won't send response headers until it receives the first message,
+    // but send_heartbeat().await waits for response headers.
     tx.send(collect_heartbeat(config, state)).await?;
-
-    // Send initial EC shards heartbeat
     tx.send(collect_ec_heartbeat(state)).await?;
 
+    let stream = tokio_stream::wrappers::ReceiverStream::new(rx);
+    let mut response_stream = client.send_heartbeat(stream).await?.into_inner();
+
     info!("Heartbeat stream established with {}", grpc_addr);
 
     let mut volume_tick = tokio::time::interval(pulse);