mux/codec/truehd: fix MLP major-sync checksum endianness (was dropping the whole TrueHD track)

Regression since the previous release, which added an MLP major-sync checksum
gate to drop genuinely-undecodable audio frames. The checksum itself was computed
with mismatched byte order: `crc16_mlp` is the correct crc_2D table (poly 0x2D,
MSB-first) but returns its two bytes in the OPPOSITE order to libavutil's
`av_crc`, and `mlp_major_sync_crc_ok` then folded in the pre-trailer word
little-endian while comparing the trailer big-endian. The net result never
matched a real major sync, so EVERY major sync was judged corrupt. That armed the
drop-forward on the first AU and, since no major sync ever validated to clear it,
collateral-dropped every following AU forever — the entire TrueHD track was
silently dropped. Its AUs then flushed only at mux end, so the track's blocks
landed physically after all the video: a decoder reading video+TrueHD had to
buffer the whole title to reach the first audio block and spiralled into an
unbounded memory runaway ("decoder ran out of memory"). Every TrueHD title
produced after the gate landed was affected; a title from the release before it
is clean. (The header-size parse — a frequent suspect for extended 7.1/Atmos
headers — is NOT the bug; it already matches ffmpeg's `mlp_get_major_sync_size`
byte-for-byte.)

Fix: compute the checksum exactly as ffmpeg's `ff_mlp_checksum16` —
`crc16_mlp(body).swap_bytes() ^ AV_RL16(word) == AV_RL16(trailer)`. Cross-verified
byte-exact against two real discs (a 7.1/Atmos title and a 5.1 title, independent
32-byte headers both validate). With the checksum correct, major syncs validate
and the drop-forward corruption protection works as intended.

Defence in depth: a major-sync checksum that STILL can't be validated (a genuinely
corrupt or as-yet-unparsed header) no longer arms the drop-forward until we hold a
validated baseline (`num_substreams` from a prior clean major sync) — so a single
bad header can never again silently drop an entire track.

Tests: the `finalize_major_sync` fixture now builds the checksum the corrected way;
a synthetic checksum-failed head major sync is kept, not dropped; the existing
baseline-then-corrupt drop-forward tests still pass. Verified end to end against a
real disc: the TrueHD track demuxes to a full, cleanly-decodable 48 kHz 8-channel
stream, interleaved with the video, instead of 0 bytes.

CHANGELOG.md

@@ -4,6 +4,20 @@
 
 ### Fixed
 
+- **TrueHD audio is no longer silently dropped (and no longer sends decoders out
+  of memory).** The previous release added an MLP major-sync checksum gate to drop
+  genuinely-undecodable audio frames, but the checksum was computed with mismatched
+  byte order — the 16-bit CRC result was folded in one endianness and compared in
+  the other — so it never validated a real major sync. The parser then judged every
+  major sync corrupt and dropped every audio frame from the first one onward,
+  flushing the whole TrueHD track at the end of the file: de-interleaved from the
+  video, which sent players and integrity checkers into an unbounded memory spiral
+  ("decoder ran out of memory"). The checksum now matches the reference
+  implementation byte-exact (cross-verified against real 7.1/Atmos and 5.1 discs),
+  so major syncs validate and only genuinely-corrupt frames are dropped; as a
+  safety net, a major sync the parser still can't validate is kept rather than
+  allowed to drop an entire track. TrueHD titles produced after the checksum gate
+  landed need a re-rip.
 - **HD DVD AACS key files are now found on every disc, not just the common
   layout.** The AACS directory and title-key filename on HD DVD are chosen by
   the authoring house, and freemkv previously assumed one fixed spelling