feat(lib): chat-room protocol (list, join, leave, say) with events

Implement the Soulseek chat-room server messages against soulfind's
wire format:
- MessageFactory builders: room-list request (64), join (14),
  leave (15), say (13).
- Handlers parsing the server side: RoomList (64) names+counts,
  JoinRoom (14) membership, SayChatroom (13), UserJoinedRoom (16),
  UserLeftRoom (17), LeaveRoom (15).
- Surface everything to the client as a drained RoomEvent stream plus
  a room-list snapshot; new Client API: request_room_list, join_room,
  leave_room, say_in_room, room_list, take_room_events.

Unit tests cover each builder and handler; server-gated e2e tests
exercise message delivery and room listing against soulfind.

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

@@ -0,0 +1,43 @@
+use crate::{
+    actor::server_actor::ServerMessage,
+    message::{Message, MessageHandler},
+};
+use std::sync::mpsc::Sender;
+
+pub struct UserLeftRoomHandler;
+
+impl MessageHandler<ServerMessage> for UserLeftRoomHandler {
+    fn get_code(&self) -> u8 {
+        17
+    }
+
+    fn handle(&self, message: &mut Message, sender: Sender<ServerMessage>) {
+        let room = message.read_string();
+        let username = message.read_string();
+        let _ = sender.send(ServerMessage::RoomUserLeft { room, username });
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn forwards_user_left() {
+        let (tx, rx) = std::sync::mpsc::channel();
+        let mut message = Message::new();
+        message.write_raw_bytes(vec![0u8; 8]);
+        message.write_string("jazz");
+        message.write_string("carol");
+        message.set_pointer(8);
+
+        UserLeftRoomHandler.handle(&mut message, tx);
+        match rx.try_recv() {
+            Ok(ServerMessage::RoomUserLeft { room, username }) => {
+                assert_eq!(room, "jazz");
+                assert_eq!(username, "carol");
+            }
+            other => panic!("unexpected: {other:?}"),
+        }
+    }
+}