`get_media_position` returned `NotConnected` whenever no media app had
been launched yet, and the 1 Hz poll folded that into "no answer", so a
connect made while paused hit LOST_POLL_MAX and disconnected after
exactly 5 s (reproduced three times on the 2026-08-29 smoke). Add a
typed `CastError::NoMediaSession`, and while idle prove liveness with
the receiver-level `get_status` request instead of the media session.
Also coalesce cast volume: a slider drag now hands the newest level to
one sender task every 120 ms instead of one Cast/SOAP round trip per
pixel.
cc
vicrodh
3a52638fce