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/demux_sink.rs

@@ -150,7 +150,7 @@ fn extension_for(codec: Codec) -> &'static str {
 }
 
 /// Short codec label for friendly filenames.
-fn codec_label(codec: Codec) -> &'static str {
+pub(crate) fn codec_label(codec: Codec) -> &'static str {
     match codec {
         Codec::Hevc => "HEVC",
         Codec::H264 => "AVC",
@@ -548,7 +548,7 @@ fn fmt_chapter_time_ns(time_secs: f64) -> String {
 }
 
 /// Serialize chapters as mkvmerge chapter XML.
-fn chapters_xml(chapters: &[Chapter]) -> String {
+pub(crate) fn chapters_xml(chapters: &[Chapter]) -> String {
     let mut s = String::new();
     s.push_str("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n");
     s.push_str("<!DOCTYPE Chapters SYSTEM \"matroskachapters.dtd\">\n");
@@ -578,7 +578,7 @@ fn chapters_xml(chapters: &[Chapter]) -> String {
 }
 
 /// Serialize chapters as OGM/simple chapter text.
-fn chapters_ogm(chapters: &[Chapter]) -> String {
+pub(crate) fn chapters_ogm(chapters: &[Chapter]) -> String {
     let mut s = String::new();
     for (i, c) in chapters.iter().enumerate() {
         let n = i + 1;