mux: reconcile fvi:// video-index sink onto rc6

Port + adapt the freemkv native per-picture video index (FVI) from the
old feat/fvi-sink branch onto rc6's codec-agnostic PictureInfo model.
This is a surgical adaptation, not a merge.

Adaptations (fvi_sink.rs, videomap.rs, tests/fvi_pipeline.rs):
- Retarget from the removed crate::mux::codec::mpeg2::PictureInfo (raw
  public fields) to rc6's authoritative crate::mux::codec::PictureInfo
  in codec/coding.rs, via its accessors.
- type from coding_type() -> CodingType{I,P,B}; emitted for ANY frame
  that carries coding (every video codec now fills it), with the
  keyframe-flag I/P fallback only when coding is absent.
- Replace the mpeg2-only tff/rff/progressive members with codec-agnostic
  members derived through the accessors: field_order (tff/bff/progressive)
  and progressive, emitted ONLY when the codec measured the signal
  (Option::Some) and omitted otherwise; plus nb_fields.
- Test fixtures rebuilt via PictureInfo::mpeg2(CodingType, Mpeg2Coding{..})
  / coding_type_only(..); added measured_cicp: None to VideoStream
  literals for rc6's struct.

Honesty decision (key / random-access):
- The codec-agnostic PictureInfo carries NO GOP-closure (no closed_gop /
  gop_start), so key is set from the frame's intra / decode-restart flag
  (frame.keyframe == coding.keyframe() for video), NOT a fabricated
  clean-RAP claim. The old gop member is honestly omitted. FVI_FORMAT.md
  is updated to document this as a limitation: key is an intra picture /
  parser-flagged decode-restart point; MPEG-2 open-GOP clean-RAP precision
  (closed_gop) is not currently distinguished. §7.1 rewritten for the
  new field_order/progressive/nb_fields members.

Wiring:
- mux/mod.rs: pub(crate) mod fvi_sink; pub(crate) mod videomap
  (#[allow(dead_code)] on videomap — the VideoMap accumulator is staged
  for side-channel reuse, sink builds records directly); pub use
  fvi_sink::FviSink.
- mux/resolve.rs: add the fvi:// output scheme to StreamUrl, parse_url,
  scheme(), path_str(), input() (write-only reject) and output()
  (constructs FviSink), mirroring the mkv:///demux:// patterns.

Provenance fix surfaced by the end-to-end test:
- pipelined_stream::consume_ps was dropping the PS demuxer's byte-exact
  source stamp (source: None) when rebuilding PesPacket, so PS/DVD-path
  frames reached the mux/index with no provenance (FVI src null). Carry
  ps.source through, matching the TS path; the real-pipeline fvi test now
  sees the stamped src sectors.

Gate: cargo +1.86 fmt + clippy --lib -D warnings clean; cargo +1.86 test
--lib (2182 passed) and --test fvi_pipeline (2 passed); precommit.sh
libfreemkv green.

src/mux/pipelined_stream.rs

@@ -208,7 +208,11 @@ impl PipelinedPesStream {
                 continue;
             };
             let pes = PesPacket {
-                source: None,
+                // Carry the PS demuxer's byte-exact source stamp through to the
+                // codec parser, exactly as the TS path does — provenance must
+                // survive the PsPacket → PesPacket seam so the frame's `source`
+                // reaches the mux/index (FVI `src`), never reconstructed.
+                source: ps.source,
                 pid,
                 pts: ps.pts.map(|p| p as i64),
                 dts: ps.dts.map(|d| d as i64),