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/mod.rs

@@ -90,6 +90,7 @@ pub(crate) mod hevc;
 pub(crate) mod m2ts_mux;
 pub(crate) mod mkv;
 pub(crate) mod mkvstream;
+pub(crate) mod mp4;
 pub(crate) mod network;
 pub(crate) mod null;
 pub(crate) mod ps;
@@ -154,6 +155,7 @@ mod tests {
         assert_eq!(parse_url("disc://").scheme(), "disc");
         assert_eq!(parse_url("m2ts://f").scheme(), "m2ts");
         assert_eq!(parse_url("mkv://f").scheme(), "mkv");
+        assert_eq!(parse_url("mp4://f").scheme(), "mp4");
         assert_eq!(parse_url("network://h:1").scheme(), "network");
         assert_eq!(parse_url("stdio://").scheme(), "stdio");
         assert_eq!(parse_url("iso://f").scheme(), "iso");