fix(rooms): start a room's roster from the member list, not the join echo

soulfind tells a room about a new member, the new member included, before
it sends them the room's member list. The roster was started from that
echo, so for a moment after joining a room read as holding nobody but us,
and `room users` returns on the first roster it sees: it could print only
the asking user. That is the flake in room_users_lists_who_is_in_the_room.

A join or leave now only updates a roster the server has sent in full,
which is also what room_members already promised for a room we have not
joined. The e2e test waits for the resident's own member list instead of a
fixed second, so it no longer races the resident's join either.

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

@@ -100,10 +100,12 @@ impl ClientContext {
                 // the whole board again on the next join.
                 self.room_tickers.remove(room);
             }
+            // Only a roster the server has sent in full is kept current: the
+            // joiner hears of their own arrival before the member list.
             RoomEvent::UserJoined { room, username } => {
-                let members =
-                    self.room_members.entry(room.clone()).or_default();
-                if let Err(at) = members.binary_search(username) {
+                if let Some(members) = self.room_members.get_mut(room)
+                    && let Err(at) = members.binary_search(username)
+                {
                     members.insert(at, username.clone());
                 }
             }

soulseek-rs-lib/src/client/tests/rooms.rs

@@ -39,6 +39,25 @@ fn the_room_roster_follows_the_joins_and_leaves_the_server_reports() {
     assert!(context.room_members("lobby").is_empty());
 }
 
+#[test]
+fn a_join_announced_before_the_member_list_does_not_start_a_roster() {
+    // soulfind tells a room about a joiner, the joiner included, before it
+    // sends them the member list. A roster started from that echo reads as a
+    // room holding nobody else.
+    let mut context = ClientContext::new();
+    context.apply_room_event(RoomEvent::UserJoined {
+        room: "lobby".to_string(),
+        username: "me".to_string(),
+    });
+    assert!(context.room_members("lobby").is_empty());
+
+    context.apply_room_event(RoomEvent::Joined {
+        room: "lobby".to_string(),
+        users: vec!["me".to_string(), "alice".to_string()],
+    });
+    assert_eq!(context.room_members("lobby"), ["alice", "me"]);
+}
+
 #[test]
 fn a_user_joining_twice_is_listed_once() {
     let mut context = ClientContext::new();