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/types.rs

@@ -328,6 +328,25 @@ impl UserInfo {
     }
 }
 
+/// What the server reports about one member of a room we joined, from the
+/// per-member stat vectors of a `JoinRoom` (code 14) reply.
+///
+/// A server may send shorter stat vectors than the membership list (older
+/// servers omit country codes entirely), so absent fields read as zero and an
+/// unreported country is `None` rather than an empty string.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct RoomUserStats {
+    pub username: String,
+    pub status: UserStatus,
+    pub average_speed: u32,
+    pub shared_files: u32,
+    pub shared_folders: u32,
+    /// Upload slots the member has free; zero means they are busy.
+    pub slots_free: u32,
+    /// Two-letter country code, when the server reports one.
+    pub country: Option<String>,
+}
+
 /// A public chat room advertised by the server (`RoomList`, code 64).
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct RoomInfo {