demux: enrich edition titles with codec/resolution/bitrate info

Extract the codec descriptor out of mp_format_track_metadata into a
shared demux_append_codec_desc() helper, and use it from a new
demux_compose_edition_title() composer that both demux_timeline.c and
demux_lavf.c call when building editions. Codec block is only added
when the program has exactly one stream of that kind, and fields
already present in the prefix (e.g. WxH from yt-dlp's format label)
are skipped to avoid duplication.