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/resolve.rs

@@ -55,6 +55,11 @@ pub enum StreamUrl {
     Dir { path: PathBuf },
     /// Null sink (write-only, discards data).
     Null,
+    /// Per-track elementary-stream output directory (`demux://`). A write-only
+    /// sink that fans each track of a title out to its own ES file (plus
+    /// chapters + delay metadata). Like `dir://` it targets a directory; the
+    /// CLI constructs the `DemuxSink` with full options before the mux loop.
+    Demux { dir: PathBuf },
     /// Unrecognized URL.
     Unknown { raw: String },
 }
@@ -71,6 +76,7 @@ impl StreamUrl {
             StreamUrl::Iso { .. } => "iso",
             StreamUrl::Dir { .. } => "dir",
             StreamUrl::Null => "null",
+            StreamUrl::Demux { .. } => "demux",
             StreamUrl::Unknown { .. } => "unknown",
         }
     }
@@ -83,7 +89,8 @@ impl StreamUrl {
             StreamUrl::M2ts { path }
             | StreamUrl::Mkv { path }
             | StreamUrl::Iso { path }
-            | StreamUrl::Dir { path } => path.to_str().unwrap_or(""),
+            | StreamUrl::Dir { path }
+            | StreamUrl::Demux { dir: path } => path.to_str().unwrap_or(""),
             StreamUrl::Network { addr } => addr,
             StreamUrl::Stdio | StreamUrl::Null => "",
             StreamUrl::Unknown { raw } => raw,
@@ -151,6 +158,11 @@ pub fn parse_url(url: &str) -> StreamUrl {
             path: PathBuf::from(rest),
         };
     }
+    if let Some(rest) = url.strip_prefix("demux://") {
+        return StreamUrl::Demux {
+            dir: PathBuf::from(rest),
+        };
+    }
     StreamUrl::Unknown {
         raw: url.to_string(),
     }
@@ -389,6 +401,8 @@ pub fn input(url: &str, opts: &InputOptions) -> io::Result<Box<dyn crate::pes::S
         // PES source. Mirror `null://` → write-only.
         StreamUrl::Dir { .. } => Err(crate::error::Error::StreamWriteOnly.into()),
         StreamUrl::Null => Err(crate::error::Error::StreamWriteOnly.into()),
+        // `demux://` is an output-only sink (per-track ES files); never a source.
+        StreamUrl::Demux { .. } => Err(crate::error::Error::StreamWriteOnly.into()),
         StreamUrl::Unknown { ref raw } => {
             Err(crate::error::Error::StreamUrlInvalid { url: raw.clone() }.into())
         }
@@ -446,6 +460,18 @@ pub fn output(
         // exactly the category the crate already rejects for `iso://`. The CLI
         // routes a `dir://` dest to `Disc::extract_tree` before reaching here.
         StreamUrl::Dir { .. } => Err(crate::error::Error::StreamReadOnly.into()),
+        // `demux://` with default options. The CLI constructs `DemuxSink`
+        // directly (with parsed flags) before reaching here, mirroring how a
+        // `dir://` dest is special-cased; this arm covers the bare
+        // `output()` call with the default option set.
+        StreamUrl::Demux { ref dir } => {
+            validate_file_path(dir, "demux")?;
+            Ok(Box::new(super::demux_sink::DemuxSink::create(
+                dir,
+                title,
+                &super::demux_sink::DemuxOptions::default(),
+            )?))
+        }
         StreamUrl::Unknown { ref raw } => {
             Err(crate::error::Error::StreamUrlInvalid { url: raw.clone() }.into())
         }
@@ -849,6 +875,12 @@ mod tests {
         }
         assert_eq!(parse_url("dir://x").scheme(), "dir");
         assert_eq!(parse_url("dir://x/y").path_str(), "x/y");
+        assert_eq!(parse_url("demux://out/movie/").path_str(), "out/movie/");
+        assert_eq!(parse_url("demux://x").scheme(), "demux");
+        assert!(
+            !parse_url("demux://x").is_disc_source(),
+            "demux:// is a sink, never a disc source"
+        );
         assert!(
             !parse_url("dir://x").is_disc_source(),
             "dir:// is a sink, never a disc source"