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/message/server/message_factory.rs

@@ -152,6 +152,20 @@ impl MessageFactory {
             .clone()
     }
 
+    /// Watch `username` (server code 5): the server replies with their
+    /// current status and share statistics, then pushes every later status
+    /// change as a `GetUserStatus` (code 7) until we unwatch them.
+    #[must_use]
+    pub fn build_watch_user(username: &str) -> Message {
+        Message::new().write_int32(5).write_string(username).clone()
+    }
+
+    /// Stop watching `username` (server code 6). The server sends no reply.
+    #[must_use]
+    pub fn build_unwatch_user(username: &str) -> Message {
+        Message::new().write_int32(6).write_string(username).clone()
+    }
+
     /// Ask the server (code 64) for the list of public chat rooms.
     #[must_use]
     pub fn build_room_list_request() -> Message {
@@ -424,3 +438,17 @@ fn test_build_file_search_message() {
     .to_vec();
     assert_eq!(expect, message.get_data());
 }
+
+#[test]
+fn test_build_watch_user() {
+    let message = MessageFactory::build_watch_user("bob");
+    let expect: Vec<u8> = [5, 0, 0, 0, 3, 0, 0, 0, b'b', b'o', b'b'].to_vec();
+    assert_eq!(expect, message.get_data());
+}
+
+#[test]
+fn test_build_unwatch_user() {
+    let message = MessageFactory::build_unwatch_user("bob");
+    let expect: Vec<u8> = [6, 0, 0, 0, 3, 0, 0, 0, b'b', b'o', b'b'].to_vec();
+    assert_eq!(expect, message.get_data());
+}