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/mod.rs

@@ -2,6 +2,7 @@ mod file_search_response;
 mod get_share_file_list;
 mod peer_init;
 mod place_in_queue_response;
+mod queue_upload;
 mod transfer_request;
 mod transfer_response;
 mod upload_failed;
@@ -13,6 +14,7 @@ pub use file_search_response::{
 pub use get_share_file_list::GetShareFileList;
 pub use peer_init::PeerInit;
 pub use place_in_queue_response::PlaceInQueueResponse;
+pub use queue_upload::QueueUploadHandler;
 pub use transfer_request::TransferRequest;
 pub use transfer_response::TransferResponse;
 pub use upload_failed::UploadFailedHandler;