Fix four MP4 conformance defects against the standards

**dec3 declared a 0 kbit/s AC-3 substream.** parse_dolby routes bsid < 11 to
parse_ac3, which leaves data_rate_kbps = 0 and keeps the AC-3 bsid, yet
dolby_sample_entry wrapped that config in ec-3/dec3 for any Codec::Ac3Plus track.
ETSI TS 102 366 Annex F.4 assigns ac-3/dac3 to an AC-3 bitstream and F.6 assigns
ec-3/dec3 to an Enhanced AC-3 one, so the entry now follows the SYNCFRAME that was
actually parsed, not the playlist's codec label. Computing an AC-3 data rate and
keeping ec-3 was rejected: it fixes one field while bit_stream_identification and
num_dep_sub keep misdescribing the stream. The bsid threshold is hoisted into one
constant so parser and entry-chooser cannot drift. Adjacent defect fixed in the
same box: data_rate is 13 bits from a u16 source, so push's mask WRAPPED anything
above 8191 (9000 became 808); it now saturates.

**colr tagged HLG as PQ, and PAL as BT.601.** video_colr carried a second,
drifted copy of the ColorSpace-to-CICP map: transfer 16 (PQ) for every BT.2020
stream with no HdrFormat override, and 6 (BT.601) for Bt470bg. Per ITU-T H.273
Table 3, HLG is 18 and BT.470-6 System B/G is 5 — and mkv::cicp_for_video already
got both right. video_colr now delegates to that shared resolver, so the
duplicated table is gone and cannot drift again. It keeps only its own decision
about WHETHER to emit the box, since an absent colr and an all-unspecified colr
mean the same thing per ISO/IEC 14496-12.

**Exact 24.000 / 30.000 / 60.000 fps was declared 23.976 / 29.97 / 59.94.**
detect_rate took the FIRST STD_RATES entry within 0.5 fps, and every 1000/1001
entry precedes its integer twin 0.024 fps away — a 0.1% error across the whole
track's mdhd and stts. Fixed as nearest-wins rather than by reordering the table:
reordering fixes today's table and re-breaks the moment someone appends a rate,
while nearest-wins is order-independent. Verified red here independently by
reverting to first-match, which fails exactly the two timing tests.

**ddts MultiAssetFlag was set from has_extension.** In the DTSSpecificBox
(ETSI TS 102 114) that flag signals more than one audio ASSET. A DTS-HD MA/HRA
track is one asset whose extension substream carries the XLL/XBR component, so
setting it from "an EXSS sync follows the core" sent a parser looking for a second
asset descriptor while StreamConstruction simultaneously said there was no
extension — the box contradicting itself. This module parses the core header only
and never reads the EXSS asset table, so 0 is the only honest declaration. A
StreamConstruction index for core+EXSS was deliberately NOT invented: that field
is a table lookup that could not be confirmed against the standard, and a wrong
index is worse than an under-declaration.

DTS_AMODE_LAYOUT's masks were independently re-derived against ETSI TS 102 114
§5.3.1 and all 16 are CORRECT — only three adjacent comments were wrong (the
AMODE 2/3/4 annotations were rotated by one, and AMODE 9's said "5.1 with LFE"
when 0x0007 is the 5.0 mask and LFE is OR'd in separately). Comments corrected.

Every one of the eight new tests decodes the field back OUT of the emitted bytes —
data_rate from the dec3 body's leading 13 bits, MultiAssetFlag from bit 48 of the
ddts tail, colr from the nclx payload inside a real stsd, and the frame rate from
mdhd.timescale plus stts.sample_delta of a fully muxed MP4 — rather than
restating arithmetic. This audit has already caught one of my own tests doing the
latter.

NOT fixed, root-caused and recorded at the site instead: an A_PCM/INT/BIG track
ships with no BitDepth, which the Matroska Codec Specifications make a MUST. The
width exists on disc (BD LPCM signals it in the ES header byte 3, DVD in the IFO
audio attribute byte 1) but neither source reaches MkvTrack::audio, and the fix
needs a new AudioStream member plus a deferred setter in files another agent held
this round. Guessing 16 was rejected — it would confidently misdecode every
24-bit disc — as was refusing the track, which would regress the 16-bit majority
that currently plays by accident.

src/mux/mkv.rs

@@ -550,6 +550,25 @@ impl MkvTrack {
             field_duration_ns: 0,
             sample_rate: sr,
             channels: ch,
+            // KNOWN GAP, not a deliberate omission. The Matroska Codec
+            // Specifications say of `A_PCM/INT/BIG` and `A_PCM/INT/LIT` that "the
+            // audio bit depth MUST be read and set from the `BitDepth` element",
+            // and raw PCM has no in-band header left to recover it from once
+            // `LpcmParser` has stripped the BD/DVD framing — so a `Codec::Lpcm`
+            // track written with 0 here (the serializer then omits BitDepth
+            // entirely) has no recoverable sample width.
+            //
+            // The value is NOT available at this call site: [`AudioStream`] has no
+            // bit-depth member, and neither of the two places that know it reaches
+            // here. BD LPCM signals it in the 4-byte ES header (byte 3 bits 7-6)
+            // that `LpcmParser` discards, and the Blu-ray clip info does not carry
+            // it at all; DVD LPCM signals it in the IFO audio-attribute block
+            // (byte 1 bits 7-6, quantization: 16/20/24-bit), which `parse_audio_attr`
+            // does not read. Closing it means carrying the depth on `AudioStream`
+            // for the DVD path and a deferred setter fed from the first PES for the
+            // BD path (the route `hdr10` and `field_order` already take). Guessing
+            // 16 here would be worse than omitting: it turns a track a player may
+            // probe or reject into one it confidently misdecodes.
             bit_depth: 0,
             dv_config: None,
             hdr10: None,