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/actor/server_actor.rs

@@ -18,6 +18,7 @@ use crate::message::server::ReloggedHandler;
 use crate::message::server::SayChatroomHandler;
 use crate::message::server::UserJoinedRoomHandler;
 use crate::message::server::UserLeftRoomHandler;
+use crate::message::server::WatchUserHandler;
 use crate::message::server::WishListIntervalHandler;
 use crate::message::server::{
     GetUserStatsHandler, GetUserStatusHandler, RoomListHandler,
@@ -202,6 +203,17 @@ pub enum ServerMessage {
         status: u32,
         privileged: bool,
     },
+    /// A `WatchUser` (code 5) reply: the initial snapshot for a user we just
+    /// started watching. The stats are absent when the server does not know
+    /// the username.
+    WatchedUserReceived {
+        username: String,
+        exists: bool,
+        status: Option<u32>,
+        average_speed: Option<u32>,
+        shared_files: Option<u32>,
+        shared_folders: Option<u32>,
+    },
     /// A `GetUserStats` (code 36) reply.
     UserStatsReceived {
         username: String,
@@ -359,6 +371,7 @@ impl ServerActor {
         handlers.register_handler(ReloggedHandler);
         handlers.register_handler(RoomListHandler);
         handlers.register_handler(GetUserStatusHandler);
+        handlers.register_handler(WatchUserHandler);
         handlers.register_handler(GetUserStatsHandler);
         handlers.register_handler(JoinRoomHandler);
         handlers.register_handler(LeaveRoomHandler);
@@ -464,6 +477,23 @@ impl ServerActor {
                     privileged,
                 });
             }
+            ServerMessage::WatchedUserReceived {
+                username,
+                exists,
+                status,
+                average_speed,
+                shared_files,
+                shared_folders,
+            } => {
+                self.forward_to_client(ClientOperation::WatchedUserReceived {
+                    username,
+                    exists,
+                    status,
+                    average_speed,
+                    shared_files,
+                    shared_folders,
+                });
+            }
             ServerMessage::UserStatsReceived {
                 username,
                 average_speed,