feat: browse another user's shared files

Add Client::browse_user (sends GetShareFileList, code 4, connecting first if
needed via the same queue-and-flush path as search replies) and
take_browse_result. A new SharedFileListResponseHandler parses the peer's
code-5 reply into SharedDirectory entries, forwarded as
ClientOperation::BrowseResult and stored per user. Verified end to end: a
second real client browses a sharer and receives its file listing.

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

@@ -17,7 +17,8 @@ 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,
+    SharedDirectory, SharedFileListResponseHandler, build_shared_file_list,
+    parse_shared_file_list,
 };
 pub use transfer_request::TransferRequest;
 pub use transfer_response::TransferResponse;

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

@@ -2,8 +2,10 @@
 //! sent in reply to `GetShareFileList` (code 4). The payload is zlib-compressed
 //! and groups files by their virtual directory.
 
-use crate::message::Message;
+use crate::message::{Message, MessageHandler};
+use crate::peer::PeerMessage;
 use crate::utils::zlib::{compress_stored, deflate};
+use std::sync::mpsc::Sender;
 
 /// One shared directory and the files directly in it (basename + size).
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -12,6 +14,18 @@ pub struct SharedDirectory {
     pub files: Vec<(String, u64)>,
 }
 
+/// Receives a peer's `SharedFileListResponse` (peer code 5) when browsing them.
+pub struct SharedFileListResponseHandler;
+impl MessageHandler<PeerMessage> for SharedFileListResponseHandler {
+    fn get_code(&self) -> u8 {
+        5
+    }
+    fn handle(&self, message: &mut Message, sender: Sender<PeerMessage>) {
+        let directories = parse_shared_file_list(message);
+        let _ = sender.send(PeerMessage::ShareListReceived(directories));
+    }
+}
+
 /// Build a `SharedFileListResponse` (peer code 5) from the directory listing.
 #[must_use]
 pub fn build_shared_file_list(dirs: &[SharedDirectory]) -> Message {

soulseek-rs-lib/src/message/server/message_factory.rs

@@ -122,6 +122,12 @@ impl MessageFactory {
             .clone()
     }
 
+    /// Ask a peer for their shared-file listing (peer code 4, no body).
+    #[must_use]
+    pub fn build_get_share_file_list() -> Message {
+        Message::new().write_int32(4).clone()
+    }
+
     #[must_use]
     pub fn build_queue_upload_message(filename: &str) -> Message {
         Message::new()