feat(lib): watch and unwatch users (WatchUser 5 / UnwatchUser 6)

Adds the server-side watch subscription a buddy list needs: WatchUser (5)
asks the server to keep pushing a user's status, UnwatchUser (6) cancels it.
The reply to a watch carries the user's current status and share statistics,
which merge into the existing per-user snapshot read via Client::user_info.

Issue #47 asked for AddUserList (141) / WatchedUserAdded (142) /
WatchedUserRemoved (143). Those codes are EnableRoomInvitations,
ChangePassword and AddRoomOperator in the Soulseek protocol -- there are no
such messages on the wire. Watching is codes 5 and 6, so that is what this
implements.

- message/server/watch_user.rs: WatchUserHandler (code 5), tolerant of an
  absent user and of a truncated frame
- MessageFactory::build_watch_user / build_unwatch_user
- Client::watch_user / unwatch_user / watched_users
- The watch reply has no privileged flag, so it preserves whatever an
  earlier GetUserStatus recorded rather than asserting false

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

@@ -473,6 +473,26 @@ impl Client {
                             error!("[client] UserStatsReceived write: {}", e);
                         }
                     },
+                    ClientOperation::WatchedUserReceived {
+                        username,
+                        exists,
+                        status,
+                        average_speed,
+                        shared_files,
+                        shared_folders,
+                    } => match client_context.write_safe() {
+                        Ok(mut ctx) => ctx.apply_watched_user(
+                            username,
+                            exists,
+                            status,
+                            average_speed,
+                            shared_files,
+                            shared_folders,
+                        ),
+                        Err(e) => {
+                            error!("[client] WatchedUserReceived write: {}", e);
+                        }
+                    },
                     ClientOperation::RoomEvent(event) => {
                         match client_context.write_safe() {
                             Ok(mut ctx) => ctx.apply_room_event(event),