Harden mux + decrypt paths; fail-loud on unresolvable keys

mp4 demuxer (untrusted input): bound every allocation sized from a box
field (stsz/stco/stsc counts, stts/ctts run-lengths, per-sample and moov
sizes, plus an absolute cap so a sparse file can't inflate file_len);
guard the parse_stsd slice and a zero mdhd timescale; cap track count so
the per-track PID can't overflow; rewrite read_moov to handle size==0 /
size<8 / 64-bit largesize; parse esds/AudioSpecificConfig for AAC; write
tkhd duration in the movie timescale.

decrypt: resolve_mux_key_map now fails loud on an extent no key can
classify instead of inheriting the previous extent's key, so a keymap
never silently carries a wrong key; the sweep/patch key-fetch recovery
fails loud when a unit is still unresolved after the retry.

AACS: reject inverted forensic segments in both range builders; compare
the forensic index in u16 space so an out-of-range value can't truncate
onto a valid u8 index. RECOVERED_ERROR no longer latches the damage zone,
preserving the 30s wedge cooldown for a following hard error.

audio: AAC/MP2/MP3/FLAC carry the last PTS across a PES with no timestamp;
the DTS-HD extension-sync search is bounded to after the core; the MP4
16.16 sample-rate field saturates. demux_sink records the video reference
before the kind filter so audio:// / sub:// keep multi-clip PTS continuity
and the DELAY tag.

Remove a dead error variant and the AACS-unsupported-video code; codec
comments cite the primary format specs; assorted doc/naming fixes and
regression tests throughout.

src/mux/timeline.rs

@@ -23,7 +23,8 @@ pub(crate) const DISCONTINUITY_GAP_NS: i64 = 1_000_000;
 /// one concatenated sector stream (clip boundaries / mpls connection_condition
 /// are not plumbed to the mux), so at a non-seamless boundary the source PES
 /// PTS jumps backward. Left uncorrected, that produces a sustained band of
-/// non-monotonic block timestamps (ffmpeg then derives non-monotonic DTS).
+/// non-monotonic block timestamps (a downstream muxer then derives
+/// non-monotonic DTS from them).
 ///
 /// A single running `offset_ns` is applied to EVERY track, so the concatenated
 /// clips form one monotonic timeline AND A/V sync is preserved (all tracks at a