joiner-catches-up-by-asking

The state a relay pushes to a new member is the source's last command replayed, so its position is however old that command is — obeying it drops the joiner at the start of something everyone else is halfway through. Ask for the live position the moment you are in, and again whenever this client rejoins the shared timeline. Use when a member who joins mid-session starts from the beginning, or restarts at whatever…