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/message/peer/queue_upload.rs

@@ -0,0 +1,19 @@
+use crate::{
+    message::{Message, MessageHandler},
+    peer::PeerMessage,
+};
+use std::sync::mpsc::Sender;
+
+/// A peer asking to download one of our shared files (peer code 43).
+pub struct QueueUploadHandler;
+
+impl MessageHandler<PeerMessage> for QueueUploadHandler {
+    fn get_code(&self) -> u8 {
+        43
+    }
+
+    fn handle(&self, message: &mut Message, sender: Sender<PeerMessage>) {
+        let filename = message.read_string();
+        let _ = sender.send(PeerMessage::IncomingQueueUpload(filename));
+    }
+}