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/disc/dvd.rs

@@ -57,7 +57,6 @@ impl Disc {
                 // PipelinedPesStream/DiscStream into the output title (mirroring
                 // the existing `codec_private` handshake). Until then `None`
                 // means the muxer falls back to TFF (correct for ~all DVDs).
-                top_field_first: None,
                 // TODO(spec): DVD MPEG-2 carries no VUI; the colour signalling is
                 // the sequence_display_extension colour_description when present.
                 // Surface it from `Mpeg2Parser` (same handshake as above) and set