MediaServer gains a RangeSource entry whose bytes may still be arriving
(known final size, blocking reads) and serves every request on its own
thread so a slow media body no longer stalls the HEAD probe or a seek's
second Range request. Chromecast: play/pause/stop never launch the
Default Media Receiver (an app launched with nothing loaded sat IDLE and
read as "track ended"); disconnect stops the receiver app instead of
leaving it on screen with the last item.
cc