Presence

Which conversation surfaces are working right now — the rail's green light.

ONE READ FOR THE WHOLE RAIL. A chat stream tells the mounted conversation that its desk is typing; it can never tell the six OTHER rows in the sidebar anything, because nothing is listening there. So presence is answered where it is actually known — the runs table — and the browser asks one question about every place at once instead of opening six streams to find out.

IT REPORTS WORK, NEVER AVAILABILITY. "Online" is true of a server every second of every day, which makes it furniture; LIVE_RUN_STATES is the desk mid-task, which is news. A run parked on a human is not in that set on purpose — see the store.