Two write-only file sinks that ignore the PES stream and emit the title
metadata at construction (fvi-style, wired through output()):
- chapters:// — chapter markers in the format the extension picks: .xml
(Matroska, default), .txt/.ogm (OGM simple), .vtt (WebVTT). Reuses the
existing chapters_xml/chapters_ogm writers; adds a WebVTT writer.
- json:// — one title's model (playlist, duration, size, format, streams,
chapters) as pretty JSON via serde_json.
New mux/meta_sink.rs; StreamUrl gains Chapters/Json; codec_label +
chapters_xml/ogm promoted to pub(crate) for reuse.
Tests: chapters_format_selected_by_extension, title_json_carries_streams_and_chapters.
