mux/mp4: DTS audio (dtsc/dtsh + ddts)

Parse the DTS core header (SFREQ/AMODE/RATE/LFF/NBLKS/FSIZE) → a ddts box
(sample rate, channel layout mask, core size, computed bitrate, LFE);
whole access units (core + DTS-HD extension substreams) pass through, so
an HD decoder finds the extension. dtsh when an extension sync follows the
core, else dtsc. Fit oracle now carries DTS / DTS-HD MA / DTS-HD HR.

Validated on 300 (real DTS-HD MA 7.1): freemkv's mp4 DTS track is
byte-identical to ffmpeg -c copy under ffprobe (dts / 48000 / 8ch / 7.1)
and decodes clean (exit 0). Channel layout correct.

src/mux/mp4/mod.rs

@@ -12,9 +12,10 @@
 //! ## Track model
 //!
 //! One video track (HEVC / H.264) plus every audio track whose codec has a clean
-//! MP4 mapping (AC-3 → `ac-3`/`dac3`, E-AC-3 → `ec-3`/`dec3`). This is the fit
-//! oracle: a codec MP4 can't carry (TrueHD, DTS, LPCM) or that has no sample
-//! entry here is **excluded, never silently dropped** — [`fit_report`] lets the
+//! MP4 mapping (AC-3 → `ac-3`/`dac3`, E-AC-3 → `ec-3`/`dec3`, DTS/DTS-HD →
+//! `dtsc`/`dtsh`/`ddts`). This is the fit oracle: a codec MP4 can't carry
+//! (TrueHD, LPCM) or that has no sample entry here is **excluded, never silently
+//! dropped** — [`fit_report`] lets the
 //! CLI enumerate exactly what was left out and why. Video NALs pass through
 //! unchanged (the demux hands us length-prefixed hvcC/avcC framing — already
 //! MP4's form). Decode timestamps are derived (the pipeline carries presentation