libfreemkv: rc.5.2 SOTL video — full Windows fps, opening-GOP proof, self-sufficient log-level 3

Three Silence-of-the-Lambs (R2 PAL SD-DVD) follow-ups for rc.5.2.

SUB-TASK 1 — Windows Explorer showed 12.5 fps (half) for the 576i25 track.
Root cause: the DefaultDecodedFieldDuration (20 ms field) element rc.5.1
added to "fix" Windows fps did the opposite. With FlagInterlaced=1 +
DefaultDuration=40 ms + DefaultDecodedFieldDuration=20 ms, Explorer halved
to 12.5 fps and MediaInfo flipped to VFR. MakeMKV's correct rip omits the
field-duration element, keeps FlagInterlaced=1 + FieldOrder=TFF +
full-frame DefaultDuration (40 ms), and Explorer shows 25 fps / MediaInfo
CFR. Fix: MkvTrack::video now passes field_duration_ns == 0 so the element
is no longer written; the 1/DefaultDuration = 25 fps signal (the only one
tools trust) is the full-frame value. Interlace signalling (FlagInterlaced,
FieldOrder=TFF) is retained — MediaInfo reads scan type from the MPEG-2 ES
picture coding extension, so it still reports Interlaced / Top Field First.
Tests pin the new TrackEntry elements (element present/absent + values).

SUB-TASK 2 — opening "menu"/still-frame video. Traced the MPEG-2
opening-GOP path; the wrong/last seq header and PTS-floor-to-0 hypotheses
are RULED OUT with file:line evidence: codecPrivate is the FIRST sequence
header (read once at headers-ready, mkvstream.rs:115 + pipelined_stream.rs:289),
DVD VOBU structure guarantees each title opens on seq header + I-frame (no
mid-GOP open), the parser back-anchors leading still-frames to the disc's
real timeline (mpeg2.rs:296-303), and the muxer anchors base on the opening
keyframe's real PTS so the t=0 floor (mkv.rs:963) never corrupts it.
Regression tests pin all three (parser + muxer level).

SUB-TASK 3 — make --log-level 3 self-sufficient (diag.rs + minimal hooks).
(a) dump the ACTUAL MKV TrackEntry elements written per track
(tag=mkv.track: FlagInterlaced, FieldOrder, DefaultDuration, field duration,
Display dims, codecPrivate hex) so Windows-fps-class metadata is verifiable
from a log alone. (b) capture the first ~100 coded frames per track (raw)
to <output>.opening.bin with a per-frame summary line (tag=mkv.opening.frame:
track, key/delta, size, PTS) so opening-GOP/menu issues are diagnosable from
a future log without the disc. Both gated to log-level 3; normal runs open
no side file and record nothing.

CI gate (Rust 1.86): fmt --check, clippy -D warnings, and test --tests all
green.

src/mux/mkvstream.rs

@@ -95,6 +95,18 @@ impl MkvStream {
     /// Create for writing PES frames → MKV container.
     /// Codec privates come from title.codec_privates (populated by input stream).
     pub fn create(writer: Box<dyn WriteSeek + Send>, title: &DiscTitle) -> io::Result<Self> {
+        Self::create_at(writer, title, None)
+    }
+
+    /// As [`create`](Self::create), but `output_path` (when known) enables the
+    /// `--log-level 3` opening-frame capture to `<output>.opening.bin`. A `None`
+    /// path (e.g. an in-memory / stdio sink) silently skips the side-file
+    /// capture; the per-track TrackEntry dump still fires.
+    pub fn create_at(
+        writer: Box<dyn WriteSeek + Send>,
+        title: &DiscTitle,
+        output_path: Option<&std::path::Path>,
+    ) -> io::Result<Self> {
         let mut tracks = Vec::new();
         let mut has_default_video = false;
         let mut has_default_audio = false;
@@ -118,7 +130,15 @@ impl MkvStream {
             tracks.push(track);
         }
 
-        let muxer = MkvMuxer::new(
+        // --log-level 3: dump the ACTUAL TrackEntry elements about to be written
+        // (FlagInterlaced / FieldOrder / DefaultDuration / DefaultDecodedFieldDuration
+        // / Display dims / codecPrivate hex) so the Windows-fps-class metadata is
+        // verifiable from a log alone. No-op when diag is off.
+        for (i, track) in tracks.iter().enumerate() {
+            crate::diag::dump_mkv_track((i + 1) as u64, track);
+        }
+
+        let mut muxer = MkvMuxer::new(
             writer,
             &tracks,
             Some(&title.playlist),
@@ -126,6 +146,13 @@ impl MkvStream {
             &title.chapters,
         )?;
 
+        // --log-level 3: capture the first ~100 coded frames per track to
+        // `<output>.opening.bin`. Only opens the side file when diag is on AND a
+        // real output path is known; otherwise it's a no-op the muxer never sees.
+        if let Some(path) = output_path {
+            muxer.set_opening_capture(crate::diag::OpeningCapture::new(path, tracks.len()));
+        }
+
         Ok(Self {
             disc_title: title.clone(),
             mode: Mode::Write {