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/server/message_factory.rs

@@ -142,6 +142,23 @@ impl MessageFactory {
             .write_string(filename)
             .clone()
     }
+    /// A TransferRequest (peer code 40) initiating an *upload*: we offer a file
+    /// to a peer who queued it, quoting our transfer token and its size.
+    #[must_use]
+    pub fn build_upload_transfer_request(
+        filename: &str,
+        token: u32,
+        size: u64,
+    ) -> Message {
+        Message::new()
+            .write_int32(40)
+            .write_int32(1) // direction: upload
+            .write_int32(token)
+            .write_string(filename)
+            .write_int64(size)
+            .clone()
+    }
+
     #[must_use]
     pub fn build_transfer_response_message(transfer: Transfer) -> Message {
         Message::new()
@@ -199,6 +216,21 @@ fn test_build_login_message() {
     assert_eq!(expect, message.get_data());
 }
 
+#[test]
+fn test_build_upload_transfer_request() {
+    use crate::types::Transfer;
+    let message =
+        MessageFactory::build_upload_transfer_request("song.mp3", 555, 4096);
+    // Decode via the production Transfer parser (dispatcher starts at offset 8).
+    let mut decoded = Message::new_with_data(message.get_buffer());
+    decoded.set_pointer(8);
+    let transfer = Transfer::new_from_message(&mut decoded);
+    assert_eq!(transfer.direction, 1); // upload
+    assert_eq!(transfer.token, 555);
+    assert_eq!(transfer.filename, "song.mp3");
+    assert_eq!(transfer.size, 4096);
+}
+
 #[test]
 fn test_build_peer_init_message() {
     use crate::peer::ConnectionType;