Publish fired a group SendAsync per event and returned, so a slow remote
viewer accumulated unbounded pending sends plus cloned MxEvents. Hub delivery
now goes through a 256-deep DropOldest channel per hub-watched session with a
single drain task and time-bounded sends. The registry counts hub viewers
separately and raises HubViewersReleased so the pump is torn down with the
last remote viewer; in-process-only sessions start no pump.
Joseph Doherty