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.
With -r <name>, ohm_upnp controls the receiver over UPnP: discovered by
friendly name via SSDP at time of use, woken, switched to its Songcast
source and pointed at this sender when a session starts; volume follows
the AirPlay slider capped at the device's VolumeLimit; standby follows
some minutes after the session ends.
