demux: add demux:// per-track elementary-stream sink

New write-only pes::Stream sink that taps the per-track PesFrame stream
(the seam right before MKV muxing) and writes each track to its own
elementary-stream file, plus chapters and per-audio-track delay metadata.
Purely additive — the MKV mux path is untouched.

- mux/demux_sink.rs: DemuxSink + EsWriter dispatch. Pass-through for
  codecs whose Frame.data is already standalone ES (MPEG-2, VC-1, AC3/
  E-AC3, DTS/DTS-HD, TrueHD, LPCM). Non-trivial writers:
  - AnnexBWriter: reframes hvcC/avcC 4-byte-length-prefixed NALs to
    Annex-B and prepends VPS/SPS/PPS parsed out of the codec_private
    configuration record (HEVC .hevc / H.264 .h264).
  - PgsSupWriter: rebuilds the HDMV 'PG' segment framing the parser
    strips, with 90kHz PTS/DTS (.sup).
  - VobSubWriter: writes raw SPUs to .sub and synthesizes the .idx
    sidecar (palette + per-SPU timestamp/filepos).
  - Delay-in-filename (mkvmerge-readable 'DELAY <n>ms') + chapter XML/OGM
    export. TimelineRebase ports the MKV muxer's seamless-branch epoch
    logic so per-track ES timestamps stay continuous across clip joins.
- mux/resolve.rs: StreamUrl::Demux variant + scheme/path_str/parse_url/
  input(write-only)/output arms.
- mux/mod.rs: module + public type re-exports.

16 unit tests: Annex-B reframing, hvcC/avcC param extraction, delay
sign/rounding + mkvmerge-regex match, PGS .sup framing, VobSub .idx
synthesis, chapter XML/OGM, timeline rebase, and end-to-end file-keying
by track + track selection.

src/mux/mod.rs

@@ -41,6 +41,7 @@ pub mod resolve;
 // dead-code lint rather than delete still-relevant scaffolding.
 #[allow(dead_code)]
 pub(crate) mod codec;
+pub(crate) mod demux_sink;
 #[allow(dead_code)]
 pub(crate) mod demux_thread;
 
@@ -90,6 +91,7 @@ pub(crate) mod stdio;
 pub(crate) mod ts;
 pub(crate) mod tsmux;
 
+pub use demux_sink::{ChaptersFmt, DelayMode, DemuxOptions, DemuxSink, Naming};
 pub use disc::DiscStream;
 pub use m2ts::M2tsStream;
 pub use mkvstream::MkvStream;
@@ -135,6 +137,7 @@ mod tests {
         assert_eq!(parse_url("stdio://").scheme(), "stdio");
         assert_eq!(parse_url("iso://f").scheme(), "iso");
         assert_eq!(parse_url("null://").scheme(), "null");
+        assert_eq!(parse_url("demux://out/").scheme(), "demux");
         assert_eq!(parse_url("bogus://x").scheme(), "unknown");
     }