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/network.rs

@@ -355,7 +355,6 @@ mod tests {
                     display_aspect: None,
                     secondary: false,
                     label: "Main".into(),
-                    top_field_first: None,
                     measured_cicp: None,
                 }),
                 Stream::Audio(AudioStream {
@@ -406,6 +405,8 @@ mod tests {
             .unwrap()
             .meta(&dt);
         let frame = pes::PesFrame {
+            coding: None,
+            source: None,
             track: 0,
             pts: 90000,
             keyframe: true,
@@ -516,6 +517,8 @@ mod tests {
         let h = std::thread::spawn(move || {
             let mut ns = NetworkStream::accept_from(listener).unwrap();
             let frame = pes::PesFrame {
+                coding: None,
+                source: None,
                 track: 0,
                 pts: 0,
                 keyframe: true,
@@ -568,6 +571,8 @@ mod tests {
             .meta(&dt);
         for i in 0..5u8 {
             let frame = pes::PesFrame {
+                coding: None,
+                source: None,
                 track: (i % 2) as usize,
                 pts: i as i64 * 90_000,
                 keyframe: i == 0,