mpv's http-header-fields string-list parser splits on commas inside
header values, so X-Plex-Device on Apple hardware (model ids like
Mac17,9) produced a colon-less garbage header line that Plex rejects
with 400 'Error parsing HTTP request' — first hit by music direct play
on macOS, latent in the mpv video path too. Deliver headers via
change-list append items (open) and per-entry -clr/-append loadfile
options (gapless arming), each item verbatim and %len%-quoted against
the outer option-list split.
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