Gapless playback still gapped between tracks on network streams: mpv
only opened the armed next track's stream at the moment the current one
ended, so any server whose connect+probe outlasts the audio output's
buffered tail (~0.5s) - remote Plex over TLS, a transcode session
starting up - produced an audible dropout at every transition.
Enable mpv's prefetch-playlist when arming a network track so the open
happens while the current track still plays. Measured on a Pixel 7 the
boundary goes from 60-233ms of inline network work to 9-16ms with no
network activity at all; a failed or superseded prefetch falls back to
the old boundary open. Local fdclose:// arms keep prefetch off: an
early open would consume the fd while playlist-pos still reads 0,
breaking _clearArmedNext's "provably never opened" close proof.
close#1869
edde746
49c117db83