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/tests.rs

@@ -622,3 +622,53 @@ fn a_replayed_transfer_response_does_not_start_a_second_transfer() {
         "a replayed TransferResponse must not dial the peer again"
     );
 }
+
+#[test]
+fn room_member_stats_are_returned_sorted_and_scoped_to_their_room() {
+    use crate::types::{RoomUserStats, UserStatus};
+    let stat = |username: &str| RoomUserStats {
+        username: username.to_string(),
+        status: UserStatus::Online,
+        average_speed: 1,
+        shared_files: 2,
+        shared_folders: 3,
+        slots_free: 1,
+        country: None,
+    };
+
+    let mut context = ClientContext::new();
+    context.apply_room_member_stats(
+        "jazz".to_string(),
+        vec![stat("carol"), stat("alice")],
+    );
+
+    let names: Vec<String> = context
+        .room_member_stats("jazz")
+        .into_iter()
+        .map(|s| s.username)
+        .collect();
+    assert_eq!(names, vec!["alice", "carol"]);
+    assert!(context.room_member_stats("unjoined").is_empty());
+}
+
+#[test]
+fn rejoining_replaces_the_previous_member_stats() {
+    use crate::types::{RoomUserStats, UserStatus};
+    let stat = |username: &str| RoomUserStats {
+        username: username.to_string(),
+        status: UserStatus::Online,
+        average_speed: 1,
+        shared_files: 2,
+        shared_folders: 3,
+        slots_free: 0,
+        country: None,
+    };
+
+    let mut context = ClientContext::new();
+    context.apply_room_member_stats("jazz".to_string(), vec![stat("alice")]);
+    context.apply_room_member_stats("jazz".to_string(), vec![stat("bob")]);
+
+    let stats = context.room_member_stats("jazz");
+    assert_eq!(stats.len(), 1, "a rejoin supersedes the old snapshot");
+    assert_eq!(stats[0].username, "bob");
+}