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/h264.rs

@@ -295,13 +295,18 @@ impl CodecParser for H264Parser {
         record.push(pps.len() as u8);
         record.extend_from_slice(pps);
 
-        // ISO 14496-15 §5.3.3.1.2: for High-Profile and related profiles
-        // (profile_idc 100, 110, 122, 144) the record has 4 trailing extension
-        // bytes carrying chroma_format_idc and bit depths. Older parsers expect
-        // the record to END after the PPS for Baseline/Main/Extended — do NOT
-        // append for those (strict parsers reject the extra bytes).
+        // ISO 14496-15 §5.3.3.1.2: for High-Profile and the related
+        // chroma/bit-depth-extended profiles the record has 4 trailing extension
+        // bytes carrying chroma_format_idc and the luma/chroma bit depths. The
+        // full set that mandates the extension is profile_idc ∈ {100, 110, 122,
+        // 144, 244 (High 4:4:4 Predictive), 44, 83, 86, 118, 128, 138, 139, 134,
+        // 135}. Older parsers expect the record to END after the PPS for
+        // Baseline/Main/Extended — do NOT append for those (strict parsers
+        // reject the extra bytes).
         let profile_idc = sps[1];
-        const HIGH_PROFILES: [u8; 4] = [100, 110, 122, 144];
+        const HIGH_PROFILES: [u8; 14] = [
+            100, 110, 122, 144, 244, 44, 83, 86, 118, 128, 138, 139, 134, 135,
+        ];
         if HIGH_PROFILES.contains(&profile_idc) {
             if let Some((chroma_fmt, depth_luma, depth_chroma)) = parse_sps_high_profile_ext(sps) {
                 // byte 0: 111111xx — reserved(6) + chroma_format_idc(2)
@@ -1464,6 +1469,36 @@ mod tests {
         );
     }
 
+    /// ISO 14496-15 §5.3.3.1.2 regression: profile_idc=244 (High 4:4:4
+    /// Predictive) ALSO mandates the chroma/bit-depth extension. It was missing
+    /// from HIGH_PROFILES, so a 244 stream took the Baseline/Main path and
+    /// emitted an avcC with NO extension bytes — non-conforming, and strict
+    /// parsers then assume 8-bit 4:2:0. The extension must be appended.
+    #[test]
+    fn avcc_profile_244_appends_extension_bytes() {
+        // profile_idc=244, chroma_format_idc=3 (4:4:4), depths both 4 (12-bit).
+        let sps = build_high_profile_sps(244, 3, 4, 4);
+        let mut parser = H264Parser::new();
+        feed_sps_pps(&mut parser, &sps);
+
+        let cp = parser.codec_private().expect("avcC must be present");
+        let ext_off = sps.len() + 14;
+        assert_eq!(
+            cp.len(),
+            ext_off + 4,
+            "profile 244 avcC must have the 4 extension bytes (len={}, expected {})",
+            cp.len(),
+            ext_off + 4
+        );
+        assert_eq!(cp[ext_off] & 0x03, 3, "chroma_format_idc must be 3 (4:4:4)");
+        assert_eq!(cp[ext_off + 1] & 0x07, 4, "bit_depth_luma_minus8 must be 4");
+        assert_eq!(
+            cp[ext_off + 2] & 0x07,
+            4,
+            "bit_depth_chroma_minus8 must be 4"
+        );
+    }
+
     /// ISO 14496-15 §5.3.3.1.2 regression: a Main-Profile SPS (profile_idc=77)
     /// must NOT have the extension bytes — strict parsers reject trailing bytes
     /// for Baseline/Main/Extended profiles.