mux: codec-agnostic PictureInfo + provenance; measure field order, never guess

Carry per-picture truth and byte-exact source provenance THROUGH the stream so
the muxer (and the upcoming video index) read MEASURED facts instead of
assuming them. Honest data in, honest data out.

- codec/coding.rs: codec-agnostic PictureInfo (CodingType / FieldOrder + the
  accessors field_order/coding_type/nb_fields/progressive/keyframe). Each codec
  folds its raw signals in; consumers use only accessors, never branch on codec.
- mpeg2: builds PictureInfo from the picture coding extension and carries it +
  SourcePos (source_marks, parallel to pts_marks) on every emitted frame.
- pes / codec::Frame: additive `coding` + `source`, forwarded through the
  highway; None for audio/subtitle and the network/stdio deserialize hop.
- mkvstream: DEFER muxer construction until the first coded picture, set the
  video track's FieldOrder from the MEASURED value, THEN write the header —
  right the first time, no guess, no seek-back. An interlaced track that arrives
  with no measured order is LOGGED loudly and left UNDETERMINED, never faked.
- mkv: MkvTrack::video no longer guesses TFF (a bitstream property the scan
  cannot know is UNDETERMINED at build). Removed VideoStream::top_field_first
  (the dead scan-time guess) crate-wide.
- Tests: parser population (every PictureInfo facet + per-PES source carry) and
  mux-stream consumption (measured -> correct; missing -> UNDETERMINED, not
  faked). Two obsolete tests updated only after confirming (their own comments)
  they existed to enforce the deleted hardcoded-TFF.

src/mux/codec/pgs.rs

@@ -57,6 +57,8 @@ impl PgsParser {
         let (start_pts, data) = self.pending.take()?;
         let duration = end_pts_ns.saturating_sub(start_pts).max(0) as u64;
         Some(Frame {
+            coding: None,
+            source: None,
             pts_ns: start_pts,
             keyframe: true,
             data,
@@ -90,6 +92,8 @@ impl CodecParser for PgsParser {
                 .take()
                 .map(|(start_pts, data)| {
                     vec![Frame {
+                        coding: None,
+                        source: None,
                         pts_ns: start_pts,
                         keyframe: true,
                         data,
@@ -115,6 +119,8 @@ impl CodecParser for PgsParser {
                 let frame = match pts {
                     Some(end) => self.emit_pending(end),
                     None => self.pending.take().map(|(start_pts, data)| Frame {
+                        coding: None,
+                        source: None,
                         pts_ns: start_pts,
                         keyframe: true,
                         data,
@@ -136,6 +142,8 @@ impl CodecParser for PgsParser {
                 // Flush any prior pending undurated and skip storing this one.
                 None => {
                     out.extend(self.pending.take().map(|(start_pts, data)| Frame {
+                        coding: None,
+                        source: None,
                         pts_ns: start_pts,
                         keyframe: true,
                         data,
@@ -159,6 +167,8 @@ impl CodecParser for PgsParser {
                     // (A missing PTS falls through to the drop path below: a
                     // bitmap with no timing reference would land at 00:00:00.)
                     out.push(Frame {
+                        coding: None,
+                        source: None,
                         pts_ns: pts.unwrap_or(0),
                         keyframe: true,
                         data: pes.data.clone(),
@@ -185,6 +195,8 @@ impl CodecParser for PgsParser {
         // the final on-screen subtitle (see the module doc).
         match self.pending.take() {
             Some((start_pts, data)) => vec![Frame {
+                coding: None,
+                source: None,
                 pts_ns: start_pts,
                 keyframe: true,
                 data,
@@ -206,6 +218,7 @@ mod tests {
 
     fn make_pes(data: Vec<u8>, pts: Option<i64>) -> PesPacket {
         PesPacket {
+            source: None,
             pid: 0x1200,
             pts,
             dts: None,