mux: chapters:// and json:// metadata sinks

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.

src/mux/mod.rs

@@ -62,6 +62,7 @@ pub(crate) mod m2ts;
 /// to round-trip codec_privates that don't fit inside the underlying format).
 /// Exposed for integration tests that exercise the wire format directly.
 pub mod meta;
+pub(crate) mod meta_sink;
 
 // ── Sequential-sink muxers ──────────────────────────────────────────────────
 //