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/dvdsub.rs

@@ -43,6 +43,8 @@ impl DvdSubParser {
         if force || buf.len() >= *size {
             let (pts_ns, _, data) = self.pending.take().unwrap();
             return Some(Frame {
+                coding: None,
+                source: None,
                 pts_ns,
                 keyframe: true,
                 data,
@@ -101,6 +103,8 @@ impl CodecParser for DvdSubParser {
             let d = ((pes.data[0] as usize) << 8) | pes.data[1] as usize;
             if d < 2 {
                 out.push(Frame {
+                    coding: None,
+                    source: None,
                     pts_ns,
                     keyframe: true,
                     data: pes.data.clone(),
@@ -112,6 +116,8 @@ impl CodecParser for DvdSubParser {
         } else {
             // Too short to carry SPU_size — pass through as a lone frame.
             out.push(Frame {
+                coding: None,
+                source: None,
                 pts_ns,
                 keyframe: true,
                 data: pes.data.clone(),
@@ -222,6 +228,7 @@ mod tests {
 
     fn make_pes(data: Vec<u8>, pts: Option<i64>) -> PesPacket {
         PesPacket {
+            source: None,
             pid: 0x1200,
             pts,
             dts: None,