feat(shares): serve queued files to peers over F connections

Implement the upload path so a peer can download our shared files:
- fix the mis-coded PlaceInQueueResponse (43 -> 44) and add a real QueueUpload
  handler (code 43) -> PeerMessage::IncomingQueueUpload;
- the peer actor forwards it as ClientOperation::QueueUpload; the client looks
  the file up in its shares, mints an upload token (high half of the token
  space, disjoint from md5 download tokens), and offers it via ServeUpload ->
  build_upload_transfer_request (peer code 40, direction=upload);
- a TransferResponse for one of our serving tokens is handled by a surgical
  early-return that emits StartUpload, leaving the download path untouched;
- StartUpload resolves the downloader's address (GetPeerAddress, cached) and
  streams the file via a new peer::upload_peer::serve_file over an F connection.

Tested: build_upload_transfer_request decodes via the production Transfer
parser; serve_file streams a file correctly over a loopback F connection.

soulseek-rs-lib/src/peer/upload_peer.rs

@@ -0,0 +1,109 @@
+//! Serving a shared file to a peer over an F (file transfer) connection.
+
+use std::fs::File;
+use std::io::{self, Read, Write};
+use std::net::{TcpStream, ToSocketAddrs};
+use std::path::Path;
+use std::time::Duration;
+
+use crate::message::server::MessageFactory;
+use crate::peer::ConnectionType;
+use crate::trace;
+
+/// Connect to the downloader's file listener and stream `path`'s bytes.
+///
+/// We announce ourselves with a `PeerInit(F)` immediately followed by the raw
+/// transfer token (so it lands in the downloader's read buffer, where the
+/// download is matched by token), then the downloader sends an 8-byte
+/// START_DOWNLOAD offset before we stream the file.
+///
+/// # Errors
+/// Returns any I/O error opening the file or talking to the peer.
+pub fn serve_file(
+    host: &str,
+    port: u32,
+    own_username: &str,
+    token: u32,
+    path: &Path,
+) -> io::Result<()> {
+    let mut file = File::open(path)?;
+
+    let socket = format!("{host}:{port}")
+        .to_socket_addrs()?
+        .next()
+        .ok_or_else(|| {
+            io::Error::new(io::ErrorKind::InvalidInput, "no address")
+        })?;
+    let mut stream =
+        TcpStream::connect_timeout(&socket, Duration::from_secs(20))?;
+    stream.set_nodelay(true).ok();
+
+    // PeerInit(F) + the 4-byte token in a single write so they coalesce.
+    let mut init = MessageFactory::build_peer_init_message(
+        own_username,
+        ConnectionType::F,
+        token,
+    )
+    .get_buffer();
+    init.extend_from_slice(&token.to_le_bytes());
+    stream.write_all(&init)?;
+    stream.flush()?;
+
+    // The downloader replies with an 8-byte START_DOWNLOAD offset first.
+    let mut offset = [0u8; 8];
+    stream.read_exact(&mut offset)?;
+
+    io::copy(&mut file, &mut stream)?;
+    stream.flush()?;
+
+    // Linger so the downloader drains everything before the socket closes.
+    std::thread::sleep(Duration::from_millis(500));
+    trace!("[upload] served {} to {}:{}", path.display(), host, port);
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::serve_file;
+    use std::io::{Read, Write};
+    use std::net::TcpListener;
+
+    #[test]
+    fn serve_file_streams_the_file_over_an_f_connection() {
+        let content: Vec<u8> = (0..4096u32).map(|i| (i % 251) as u8).collect();
+        let dir = std::env::temp_dir()
+            .join(format!("soulseek-upload-{}", std::process::id()));
+        std::fs::create_dir_all(&dir).unwrap();
+        let path = dir.join("probe.bin");
+        std::fs::write(&path, &content).unwrap();
+
+        let listener = TcpListener::bind("127.0.0.1:0").unwrap();
+        let port = u32::from(listener.local_addr().unwrap().port());
+
+        let uploader = std::thread::spawn(move || {
+            serve_file("127.0.0.1", port, "me", 777, &path)
+        });
+
+        // Downloader side: accept, read PeerInit(F) frame + raw token, send the
+        // 8-byte START_DOWNLOAD offset, then read the streamed bytes.
+        let (mut stream, _) = listener.accept().unwrap();
+        let mut len_buf = [0u8; 4];
+        stream.read_exact(&mut len_buf).unwrap();
+        let len = u32::from_le_bytes(len_buf) as usize;
+        let mut payload = vec![0u8; len];
+        stream.read_exact(&mut payload).unwrap();
+        let mut token = [0u8; 4];
+        stream.read_exact(&mut token).unwrap();
+        assert_eq!(u32::from_le_bytes(token), 777);
+
+        stream.write_all(&[0u8; 8]).unwrap();
+        stream.flush().unwrap();
+
+        let mut received = Vec::new();
+        stream.read_to_end(&mut received).unwrap();
+        assert_eq!(received, content);
+
+        let _ = uploader.join();
+        let _ = std::fs::remove_dir_all(dir);
+    }
+}