Streams AirPlay audio to OpenHome/Songcast receivers as OHU unicast PCM
(16/24-bit). Sends are paced on the PTP master clock so receivers slave
to the AirPlay group clock through their buffer-fill servo; MediaLatency
carries the phase, matching the wire behaviour of Linn's own sender
(zero flags and timestamps). delay() reports the newest frame's playtime
lead so shairport feeds a receiver latency ahead. Handles Join, Listen,
Leave and Resend with a frame history, and forwards track metadata and
metatext from the metadata hub.