feat(lib): per-member room statistics from the JoinRoom reply

The JoinRoom (code 14) reply carries, after the membership list, four
parallel vectors describing each member: status, share statistics, free
upload slots and country. The handler stopped after the usernames, so all of
it was discarded. It is now parsed and exposed as Client::room_member_stats.

Issue #48 asked for RoomStats (57) and RoomListStats (55). Those codes are
UserInterests and MyRecommendations in the Soulseek protocol; there is no
RoomStats message on the wire. Per-room user counts already arrive with
RoomList (64), which the client handles, and the per-member statistics are
part of the JoinRoom reply -- so that is where this adds them.

- join_room.rs: parse the stat vectors, each bounded by the payload so a
  hostile count cannot hang or overallocate
- types::RoomUserStats; every field looked up positionally so a server
  sending short vectors (or omitting countries) does not shift the roster
- ServerMessage/ClientOperation::RoomMemberStats and the context snapshot
  behind Client::room_member_stats, replaced on each rejoin

soulseek-rs-lib/src/client/operations.rs

@@ -479,6 +479,16 @@ impl Client {
                             Err(e) => error!("[client] RoomEvent write: {}", e),
                         }
                     }
+                    ClientOperation::RoomMemberStats { room, stats } => {
+                        match client_context.write_safe() {
+                            Ok(mut ctx) => {
+                                ctx.apply_room_member_stats(room, stats);
+                            }
+                            Err(e) => {
+                                error!("[client] RoomMemberStats write: {}", e);
+                            }
+                        }
+                    }
                     ClientOperation::WishlistInterval(seconds) => {
                         if let Ok(mut ctx) = client_context.write_safe() {
                             ctx.wishlist_interval = Some(seconds);