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/disc/mod.rs

@@ -603,7 +603,12 @@ impl Codec {
             // lossless pair, parallel to 0x81/0xA1 for AC-3. 0xA2 is
             // lossless MA, not lossy HR.
             0xA2 => Codec::DtsHdMa,
-            0x90 | 0x91 => Codec::Pgs,
+            // 0x90 = Presentation Graphics (PG / subtitles). 0x91 = Interactive
+            // Graphics (IG / menus) and 0x92 = Text subtitles are distinct HDMV
+            // coding types and are NOT PG subtitle streams; only 0x90 maps to
+            // Pgs. IG (0x91) falls through to Unknown so the PMT/STN walker drops
+            // it rather than surfacing a bogus PGS subtitle track for a menu ES.
+            0x90 => Codec::Pgs,
             ct => Codec::Unknown(ct),
         }
     }
@@ -5776,6 +5781,20 @@ mod tests {
         assert_eq!(Codec::from_coding_type(0x86), Codec::DtsHdMa);
     }
 
+    /// HDMV coding_type 0x90 = Presentation Graphics (PG / subtitles) → Pgs,
+    /// but 0x91 = Interactive Graphics (IG / menus) is NOT a subtitle stream.
+    /// It must NOT map to Pgs (whose kind() is Subtitle), else a menu ES would
+    /// surface as a bogus PGS subtitle track. 0x91 falls through to Unknown so
+    /// the PMT/STN walker drops it.
+    #[test]
+    fn coding_type_ig_0x91_is_not_pgs_subtitle() {
+        assert_eq!(Codec::from_coding_type(0x90), Codec::Pgs);
+        assert_eq!(Codec::from_coding_type(0x90).kind(), CodecKind::Subtitle);
+        // IG must not be a PGS subtitle.
+        assert_eq!(Codec::from_coding_type(0x91), Codec::Unknown(0x91));
+        assert_ne!(Codec::from_coding_type(0x91).kind(), CodecKind::Subtitle);
+    }
+
     /// chapter_name emits a bare 1-based ordinal (no localized prose).
     #[test]
     fn chapter_name_is_bare_ordinal() {