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.

tests/disc_tests.rs

@@ -169,7 +169,6 @@ fn title_with_video(
             display_aspect: None,
             secondary: false,
             label: String::new(),
-            top_field_first: None,
             measured_cicp: None,
         })],
         chapters: Vec::new(),

tests/streams.rs

@@ -23,7 +23,6 @@ fn sample_disc_title() -> DiscTitle {
                 display_aspect: None,
                 secondary: false,
                 label: "Main".into(),
-                top_field_first: None,
                 measured_cicp: None,
             }),
             Stream::Audio(AudioStream {
@@ -300,6 +299,8 @@ fn m2ts_stream_write_read() {
     // Write some PES frames
     for i in 0..5u8 {
         let frame = libfreemkv::pes::PesFrame {
+            coding: None,
+            source: None,
             track: 0,
             pts: i as i64 * 1_000_000,
             keyframe: i == 0,
@@ -322,6 +323,8 @@ fn m2ts_stream_write_read() {
 fn m2ts_pes_frame_roundtrip() {
     // PesFrame serialize/deserialize roundtrip
     let frame = libfreemkv::pes::PesFrame {
+        coding: None,
+        source: None,
         track: 2,
         pts: 1_234_567_890,
         keyframe: true,
@@ -403,7 +406,6 @@ fn meta_codec_roundtrip() {
             display_aspect: None,
             secondary: false,
             label: String::new(),
-            top_field_first: None,
             measured_cicp: None,
         }));
     }
@@ -507,7 +509,6 @@ fn meta_all_stream_types() {
                 display_aspect: None,
                 secondary: false,
                 label: "Primary".into(),
-                top_field_first: None,
                 measured_cicp: None,
             }),
             Stream::Audio(AudioStream {
@@ -596,6 +597,8 @@ fn mkvstream_write_finish() {
     // since there is no real codec data, but it should not panic)
     for i in 0..20u8 {
         let frame = libfreemkv::pes::PesFrame {
+            coding: None,
+            source: None,
             track: 0,
             pts: i as i64 * 1_000_000,
             keyframe: i == 0,
@@ -658,6 +661,8 @@ fn mkvstream_roundtrip_bdts() {
     // Write PES frames targeting the audio track
     for i in 0..10u8 {
         let frame = libfreemkv::pes::PesFrame {
+            coding: None,
+            source: None,
             track: 0,
             pts: i as i64 * 1_000_000,
             keyframe: true,
@@ -694,7 +699,6 @@ fn mkvstream_meta_preserves_all_streams() {
                 display_aspect: None,
                 secondary: false,
                 label: "Main Video".into(),
-                top_field_first: None,
                 measured_cicp: None,
             }),
             Stream::Audio(AudioStream {
@@ -795,7 +799,6 @@ fn mkvstream_e2e_h264_produces_valid_mkv() {
             display_aspect: None,
             secondary: false,
             label: "Main".into(),
-            top_field_first: None,
             measured_cicp: None,
         })],
         chapters: Vec::new(),
@@ -857,6 +860,8 @@ fn mkvstream_e2e_h264_produces_valid_mkv() {
 
     // Write the ES data (SPS+PPS+IDR) as a keyframe PES frame.
     let frame1 = libfreemkv::pes::PesFrame {
+        coding: None,
+        source: None,
         track: 0,
         pts: 1_000_000_000, // 1 second in ns
         keyframe: true,
@@ -867,6 +872,8 @@ fn mkvstream_e2e_h264_produces_valid_mkv() {
 
     // Write a second non-IDR frame
     let frame2 = libfreemkv::pes::PesFrame {
+        coding: None,
+        source: None,
         track: 0,
         pts: 1_041_700_000, // ~1 frame later in ns
         keyframe: false,