mux/mp4: M2 — audio tracks + fit oracle (multi-track)

Generalize the sink to N tracks: one video + every MP4-mappable audio
track. Audio sample entries built from the first frame's bitstream —
AC-3 (ac-3/dac3) and E-AC-3 (ec-3/dec3), parsing the (E-)AC-3 BSI for
fscod/bsid/bsmod/acmod/lfeon and the data rate. Per-sample audio
durations from PTS deltas (no reorder → no ctts, all sync). Fit oracle
(mp4_fit_report, exported): video HEVC/H264, audio AC-3/E-AC-3;
TrueHD/DTS/LPCM and bitmap subs are excluded with a typed reason so the
CLI can report exclusions — never a silent drop.

Verified vs ffmpeg -c copy on real discs: AVC+AC3 and HEVC(HDR10)+AC3
both frame-exact on every track (video 2650/4270, audio 5567/3454),
duration and colour identical, clean decode. On a pathological 4-clip
title ffmpeg's OWN output emits the same DTS-monotonicity warnings (more
of them) — source-inherent, not a muxer defect.

src/mux/mod.rs

@@ -116,6 +116,7 @@ pub(crate) mod videomap;
 pub use disc::DiscStream;
 pub use m2ts::M2tsStream;
 pub use mkvstream::MkvStream;
+pub use mp4::{Mp4FitReport, Mp4SkipReason, fit_report as mp4_fit_report};
 pub use network::NetworkStream;
 pub use null::NullStream;
 pub use pipelined_stream::PipelinedPesStream;