mux: fix keyframe signalling for BlockGroup frames

Inside a Matroska BlockGroup the SimpleBlock 0x80 keyframe bit is
reserved and is always written as 0; keyframe-ness is carried only by
the presence or absence of a ReferenceBlock child. Both halves of the
round-trip got this wrong:

  - the reader skipped past ReferenceBlock and read the reserved bit,
    so every BlockGroup frame came back as a non-keyframe;
  - write_block_group discarded its `keyframe` argument and never
    emitted a ReferenceBlock, on the assumption that only intra frames
    (PGS subtitles) reached that path.

The MPEG-2 parser stamps a per-frame duration on I, P and B pictures
alike, so all MPEG-2 video is written as a BlockGroup. That makes the
assumption false and left no video frame looking like a keyframe on
read-back. Downstream, mkv:// -> m2ts:// dropped every video frame (the
TS muxer discards non-key video until the first keyframe) while still
reporting success, and mkv:// -> mkv:// and the stdio round-trip failed
E6008, because the MKV muxer opens a cluster only on a track-0 video
keyframe and so wrote nothing at all. HEVC was unaffected: it carries no
per-frame duration, so it takes the SimpleBlock path where the flag bit
is authoritative.

Verified on a real CSS DVD: 841 keyframes out of 11440 video packets
survive a re-mux, matching the I-picture count in the source bitstream.

Also stop running non-NAL video through the Annex-B converter. MPEG-2
and VC-1 elementary streams are already start-code framed, so
length-prefix conversion corrupts them. TsMuxer takes a per-track
nal_video flag, defaulting to the previous behaviour, which M2tsStream
sets from each video stream's codec.

src/mux/m2ts.rs

@@ -8,7 +8,7 @@
 //! write-only.
 
 use super::meta;
-use crate::disc::{DiscTitle, Stream as DiscStream};
+use crate::disc::{Codec, DiscTitle, Stream as DiscStream};
 use std::io::{self, Write};
 
 /// BD transport stream write sink with embedded FMKV metadata
@@ -40,6 +40,16 @@ impl M2tsStream {
             .collect();
         let boxed: Box<dyn Write + Send> = Box::new(writer);
         let mut muxer = super::tsmux::TsMuxer::new(boxed, &pids);
+        // Only HEVC/H.264 arrive length-prefixed (MKV/PES NALU convention);
+        // MPEG-2 and VC-1 are already start-code ES and must NOT go through
+        // Annex-B conversion (see `TsMuxer::set_nal_video`) or the frame is
+        // silently mangled while the mux still reports success.
+        for (i, s) in title.streams.iter().enumerate() {
+            if let DiscStream::Video(v) = s {
+                let is_nal = matches!(v.codec, Codec::Hevc | Codec::H264);
+                muxer.set_nal_video(i, is_nal)?;
+            }
+        }
         for (i, cp) in title.codec_privates.iter().enumerate() {
             // codec_privates is parallel to streams/pids; ignore any
             // trailing entries that exceed the track count rather than