mux: native progressive MP4 muxer (mp4://) — M1 video track

New mux/mp4: writes ftyp+mdat+moov (moov-at-end), streaming samples into
a 64-bit mdat and building the sample tables in memory, patched at
finish(). Video track (HEVC/AVC): passthrough length-prefixed NALs
(already MP4 framing), full stts/stsz/stsc/co64/stss and signed ctts,
CFR-derived decode timeline (pipeline carries presentation PTS only), and
a colr box for HDR10 colour signalling. hvc1/avc1 sample entry from the
hvcC/avcC codec_private. Fail-loud on codecs with no MP4 mapping.

Verified against ffmpeg -c copy on real discs: AVC-SDR (300) and
HEVC-HDR10 UHD (Dune) both frame-exact (8159 / 8160 frames), colour-exact
(bt2020/smpte2084/bt2020nc), and clean-decoding. Audio + fit oracle land
in M2.

src/mux/resolve.rs

@@ -45,6 +45,10 @@ pub enum StreamUrl {
     M2ts { path: PathBuf },
     /// Matroska container file.
     Mkv { path: PathBuf },
+    /// Progressive MP4 (ISO-BMFF) mux output (`mp4://`). Like `mkv://` but writes
+    /// a single self-contained `.mp4` (ftyp+mdat+moov). Compatibility export —
+    /// carries only MP4-mappable codecs; see `mux::mp4`.
+    Mp4 { path: PathBuf },
     /// Network stream (host:port).
     Network { addr: String },
     /// Standard I/O (stdin/stdout).
@@ -98,6 +102,7 @@ impl StreamUrl {
             StreamUrl::Disc { .. } => "disc",
             StreamUrl::M2ts { .. } => "m2ts",
             StreamUrl::Mkv { .. } => "mkv",
+            StreamUrl::Mp4 { .. } => "mp4",
             StreamUrl::Network { .. } => "network",
             StreamUrl::Stdio => "stdio",
             StreamUrl::Iso { .. } => "iso",
@@ -121,6 +126,7 @@ impl StreamUrl {
             StreamUrl::Disc { device: None } => "",
             StreamUrl::M2ts { path }
             | StreamUrl::Mkv { path }
+            | StreamUrl::Mp4 { path }
             | StreamUrl::Iso { path }
             | StreamUrl::Dir { path }
             | StreamUrl::Demux { dir: path }
@@ -169,6 +175,11 @@ pub fn parse_url(url: &str) -> StreamUrl {
             path: PathBuf::from(rest),
         };
     }
+    if let Some(rest) = url.strip_prefix("mp4://") {
+        return StreamUrl::Mp4 {
+            path: PathBuf::from(rest),
+        };
+    }
     if let Some(rest) = url.strip_prefix("network://") {
         return StreamUrl::Network {
             addr: rest.to_string(),
@@ -502,6 +513,8 @@ 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()),
+        // `mp4://` is an output-only mux sink for now (no MP4 demux source).
+        StreamUrl::Mp4 { .. } => Err(crate::error::Error::StreamWriteOnly.into()),
         // `demux://` is an output-only sink (per-track ES files); never a source.
         StreamUrl::Demux { .. }
         | StreamUrl::Video { .. }
@@ -540,6 +553,10 @@ pub fn output(
                 ));
             Ok(Box::new(MkvStream::create(writer, title, Some(path))?))
         }
+        StreamUrl::Mp4 { ref path } => {
+            validate_file_path(path, "mp4")?;
+            Ok(Box::new(super::mp4::Mp4Sink::create(path, title)?))
+        }
         StreamUrl::M2ts { ref path } => {
             validate_file_path(path, "m2ts")?;
             let writer = std::io::BufWriter::with_capacity(