feat(shares): reply to browse requests with a SharedFileListResponse (code 5)

GetShareFileList (peer code 4) previously replied with a malformed message (a
server SharedFolders stats message over a peer connection). It now emits
ShareListRequested, and the client builds a real SharedFileListResponse
(code 5) from its shares grouped by directory. Adds a std-only code-5
encoder/decoder (build_shared_file_list / parse_shared_file_list) with a
roundtrip test, and Shares::directories() to group files. The decoder is the
foundation for the Browse feature.

soulseek-rs-lib/src/message/peer/mod.rs

@@ -3,6 +3,7 @@ mod get_share_file_list;
 mod peer_init;
 mod place_in_queue_response;
 mod queue_upload;
+mod shared_file_list;
 mod transfer_request;
 mod transfer_response;
 mod upload_failed;
@@ -15,6 +16,9 @@ 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 shared_file_list::{
+    SharedDirectory, build_shared_file_list, parse_shared_file_list,
+};
 pub use transfer_request::TransferRequest;
 pub use transfer_response::TransferResponse;
 pub use upload_failed::UploadFailedHandler;