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/h264.rs

@@ -203,6 +203,8 @@ impl CodecParser for H264Parser {
         }
 
         vec![Frame {
+            coding: None,
+            source: None,
             pts_ns,
             keyframe,
             data: frame_data,
@@ -488,6 +490,7 @@ mod tests {
 
     fn make_pes(data: Vec<u8>, pts: Option<i64>) -> PesPacket {
         PesPacket {
+            source: None,
             pid: 0x1011,
             pts,
             dts: None,
@@ -825,6 +828,7 @@ mod tests {
         data.extend_from_slice(&[0x00, 0x10]);
 
         let pes = PesPacket {
+            source: None,
             pid: 0x1011,
             pts: Some(180000), // 2 seconds (presentation)
             dts: Some(90000),  // 1 second (decode)
@@ -1133,6 +1137,7 @@ mod tests {
         // PTS absent → DTS is used (or().map). pts.or(dts) per the comment.
         let mut parser = H264Parser::new();
         let pes = PesPacket {
+            source: None,
             pid: 0x1011,
             pts: None,
             dts: Some(90000),
@@ -1147,6 +1152,7 @@ mod tests {
     fn no_pts_no_dts_defaults_zero() {
         let mut parser = H264Parser::new();
         let pes = PesPacket {
+            source: None,
             pid: 0x1011,
             pts: None,
             dts: None,