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/bluray.rs

@@ -152,7 +152,6 @@ impl Disc {
                         // the measured field order (H.264/HEVC pic_struct, VC-1
                         // pulldown) from the codec parser instead of the TFF
                         // fallback; needs the parser→title channel (see dvd.rs).
-                        top_field_first: None,
                         // TODO(spec): prefer the HEVC/H.264 VUI colour_description
                         // (measured CICP) over this MPLS playlist-nibble guess
                         // once the parser surfaces it through the output title.

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

src/disc/mod.rs

@@ -196,14 +196,6 @@ pub struct VideoStream {
     pub secondary: bool,
     /// Extra label (e.g. "Dolby Vision EL")
     pub label: String,
-    /// Field-display order MEASURED from the elementary stream's interlace
-    /// signalling (MPEG-2 picture coding extension `top_field_first`, H.264/HEVC
-    /// `pic_struct`), when available: `Some(true)` = top-field-first,
-    /// `Some(false)` = bottom-field-first. `None` = not measured — the muxer
-    /// falls back to TFF for interlaced content (the dominant DVD/HD case). The
-    /// container FieldOrder must agree with the bitstream, so a measured BFF
-    /// stream must NOT be stamped TFF. Ignored for progressive video.
-    pub top_field_first: Option<bool>,
     /// CICP colour signalling (matrix, transfer, primaries, full_range) MEASURED
     /// from the bitstream — HEVC/H.264 VUI `colour_description` or MPEG-2
     /// `sequence_display_extension`. `Some(...)` takes precedence over the
@@ -3805,7 +3797,6 @@ mod tests {
                 display_aspect: None,
                 secondary: false,
                 label: String::new(),
-                top_field_first: None,
                 measured_cicp: None,
             })],
             chapters: Vec::new(),