Who comes and goes in every world on the node, told to whoever asks.

Cauldron2D.World.Presence.subscribe()
receive do
  {:cauldron_players, world, :joined, id} -> ...
  {:cauldron_players, world, :left, id} -> ...
end

A subscribed process is sent a message for every join and leave in every Cauldron2D.World until it unsubscribes or exits; world is the world's pid.