Two extraction sinks built on the existing demux machinery: audio:// keeps
only audio tracks (native containers .thd/.dts/.ac3/.eac3/.pcm...), sub://
only subtitle tracks (PGS .sup, VobSub .idx+.sub, text .srt) — one file per
track, no video, no chapters sidecar. DemuxOptions gains a kind_filter; the
DemuxSink writers (already complete) are reused verbatim. StreamUrl gains
Audio/Sub variants; output() builds the filtered DemuxSink.
Test: kind_filter_keeps_only_the_selected_class.