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/disc.rs

@@ -713,6 +713,7 @@ impl crate::pes::Stream for DiscStream {
                             continue;
                         };
                         let pes = super::ts::PesPacket {
+                            source: None,
                             pid,
                             pts: ps.pts.map(|p| p as i64),
                             dts: ps.dts.map(|d| d as i64),
@@ -775,6 +776,8 @@ impl crate::pes::Stream for DiscStream {
                             // attribute consumer-thread time to
                             // "demux + framing" vs "codec parse".
                             self.pending_frames.push_back(crate::pes::PesFrame {
+                                coding: None,
+                                source: None,
                                 track,
                                 pts: pes.pts.map(super::codec::pts_to_ns).unwrap_or(0),
                                 keyframe: false,
@@ -833,6 +836,7 @@ impl crate::pes::Stream for DiscStream {
                     };
 
                     let pes = super::ts::PesPacket {
+                        source: None,
                         pid,
                         pts: ps.pts.map(|p| p as i64),
                         dts: ps.dts.map(|d| d as i64),