fix(libfreemkv): rc6 hardening pass — mux timeline/colour/PCR, demux panic sentinel, parser robustness + doc accuracy

Surgical fixes (each with a regression test that fails without the change):

mux/mkv.rs, mux/demux_sink.rs: drive the clip-boundary timeline epoch
off the resolved PRIMARY VIDEO track, not the literal stream index 0.
An M2TS/PMT title can list an audio ES before video, so streams[0] may
be audio; a non-video epoch driver ratchets the frontier and inflates
the timeline. mkv cluster-opening falls back to track 0 for audio-only
titles so they still open clusters.

mux/codec/ac3.rs: correct ACMOD_CHANNELS — acmod=5 (3/1) is 4 channels,
not 3 (was undercounting a 3/1 stream); fix the A/52 Table 5.8 doc.

disc/mod.rs: HDMV coding_type 0x91 (Interactive Graphics / menus) no
longer maps to PGS subtitle — it falls through to Unknown so the PMT/STN
walker drops it instead of surfacing a bogus subtitle track.

mux/videomap.rs + mux/mkv.rs: FVI colour now mirrors the MKV muxer's CICP
precedence (measured CICP authoritative; HDR-driven PQ/HLG transfer
override) via a shared cicp_for_video helper, so the two sinks can't
disagree (HDR10 BT.2020 no longer emits SDR transfer 14).

mux/mkvstream.rs: saturating_add on cluster_ts + rel_ts so an adversarial
CLUSTER_TIMESTAMP near i64::MAX can't overflow/panic before the existing
saturating_mul.

mux/timeline.rs: tighten the tail-straggler clamp so a normal new-epoch
non-video frame leading the sparse video frontier by >3s is not demoted
into the previous clip's epoch.

mux/m2ts_mux/mod.rs: re-stamp PCR per video TS packet (mid-PES), not only
at PES boundaries, so a large UHD I-frame can't open a multi-second PCR
gap; modular 33-bit PTS rebasing so a real 90 kHz clock wrap is not
collapsed to PTS 0 (pre-base frames still floor to 0).

io/byte_prefetcher.rs, sector/prefetched.rs: wrap the producer feed loop
in catch_unwind and emit a typed error sentinel on panic, so a mid-stream
producer panic is not read as a clean EOF at the demux boundary (which
would silently truncate the mux).

mux/codec/h264.rs: extend HIGH_PROFILES to the full ISO/IEC 14496-15 set
that mandates the avcC chroma/bit-depth extension (adds 244 et al.).

Doc/comment accuracy: css/mod.rs (50000 sectors, not scrambled-sectors),
aacs/decrypt.rs (decrypt_unit already-clear path), ifo.rs (TT_SRPT at
0xC4), css/lfsr.rs (LFSR0 24-bit; TAB1-then-XOR cipher; real scramble-flag
predicate), disc/read_error.rs (for_sweep does bounded transient retries).

Skipped: keydb.rs SSRF guard (low/latent, no live caller) — a hard
loopback block breaks an existing behavioral test that exercises the
header-EOF path over a loopback server; a clean fix needs a resolver test
seam beyond this surgical pass. The sibling keydb_fetch.rs comment fix is
out of scope (freemkv crate).

src/mux/codec/ac3.rs

@@ -269,12 +269,12 @@ fn frame_duration_ns(data: &[u8], bsid: u8) -> u64 {
 /// Index is the 3-bit acmod value; add 1 when `lfeon` is set.
 ///
 /// ```text
-///   0 = 1+1 (Ch1, Ch2)  -> 2     4 = 3/0 (L,C,R)        -> 3
-///   1 = 1/0 (C, mono)   -> 1     5 = 2/1 (L,R,S)        -> 3
-///   2 = 2/0 (L, R)      -> 2     6 = 3/1 (L,C,R,S)      -> 4
+///   0 = 1+1 (Ch1, Ch2)  -> 2     4 = 2/1 (L,R,S)        -> 3
+///   1 = 1/0 (C, mono)   -> 1     5 = 3/1 (L,C,R,S)      -> 4
+///   2 = 2/0 (L, R)      -> 2     6 = 2/2 (L,R,SL,SR)    -> 4
 ///   3 = 3/0 (L,C,R)     -> 3     7 = 3/2 (L,C,R,SL,SR)  -> 5
 /// ```
-const ACMOD_CHANNELS: [u8; 8] = [2, 1, 2, 3, 3, 3, 4, 5];
+const ACMOD_CHANNELS: [u8; 8] = [2, 1, 2, 3, 3, 4, 4, 5];
 
 /// Decode the channel count of an (E-)AC-3 frame from its bitstream `acmod` and
 /// `lfeon`, starting at the 0x0B77 syncword. Returns `None` when the frame is
@@ -1129,12 +1129,18 @@ mod tests {
 
     #[test]
     fn acmod_channels_3_0_and_2_1() {
-        // acmod=4 (3/0 L,C,R) → 3 (exercises cmixlev present, surmixlev absent).
+        // Per A/52 Table 5.8: acmod 4 = 2/1, 5 = 3/1, 6 = 2/2.
+        // acmod=4 (2/1 L,R,S) → 3 (surmixlev present, no centre → no cmixlev).
         assert_eq!(acmod_channels(&make_bsi(4, false)), Some(3));
-        // acmod=5 (2/1 L,R,S) → 3 (surmixlev present, no centre).
-        assert_eq!(acmod_channels(&make_bsi(5, false)), Some(3));
-        // acmod=6 (3/1) + LFE → 5; lfeon position shifts after both
-        // cmixlev (centre) and surmixlev (surround) 2-bit fields.
+        // acmod=5 (3/1 L,C,R,S) → 4 (centre → cmixlev present, surround →
+        // surmixlev present). This is the regression case: index 5 was wrongly
+        // 3 in ACMOD_CHANNELS, undercounting a 3/1 stream by one channel.
+        assert_eq!(acmod_channels(&make_bsi(5, false)), Some(4));
+        // acmod=5 (3/1) + LFE → 5; lfeon position shifts after both cmixlev
+        // (centre) and surmixlev (surround) 2-bit fields.
+        assert_eq!(acmod_channels(&make_bsi(5, true)), Some(5));
+        // acmod=6 (2/2 L,R,SL,SR) → 4 (surmixlev present, no centre); +LFE → 5.
+        assert_eq!(acmod_channels(&make_bsi(6, false)), Some(4));
         assert_eq!(acmod_channels(&make_bsi(6, true)), Some(5));
     }