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/actor/peer_actor.rs

@@ -38,6 +38,8 @@ pub enum PeerMessage {
     RequestTransfer(Download),
     /// A peer queued one of our shared files for download (they sent us code 43).
     IncomingQueueUpload(String),
+    /// A peer asked to browse our shared files (they sent us code 4).
+    ShareListRequested,
     /// Offer the queued file to that peer: send an upload TransferRequest.
     ServeUpload {
         token: u32,
@@ -347,6 +349,15 @@ impl PeerActor {
                 );
                 self.send_message(message);
             }
+            PeerMessage::ShareListRequested => {
+                let requester_key = self.peer_username();
+                if let Err(e) = self
+                    .client_channel
+                    .send(ClientOperation::ShareListRequested { requester_key })
+                {
+                    error!("[peer_actor] forward ShareListRequested: {}", e);
+                }
+            }
             PeerMessage::RequestTransfer(download) => {
                 let message = MessageFactory::build_transfer_request_message(
                     &download.filename,