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/mod.rs

@@ -52,6 +52,10 @@ pub(crate) mod demux_thread;
 // network/stdio implementations that no external caller had business
 // reaching for.
 pub(crate) mod ebml;
+/// `fvi://` sink — freemkv's native per-picture video index (see
+/// `docs/FVI_FORMAT.md`). A write-only PES sink that emits one JSON-Lines record
+/// per coded picture; reuses the pure-data [`videomap`] model.
+pub(crate) mod fvi_sink;
 pub(crate) mod m2ts;
 /// FMKV metadata header (used by `M2tsStream` / `NetworkStream` / `StdioStream`
 /// to round-trip codec_privates that don't fit inside the underlying format).
@@ -93,9 +97,18 @@ pub(crate) mod stdio;
 pub(crate) mod timeline;
 pub(crate) mod ts;
 pub(crate) mod tsmux;
+/// Reusable, pure-data per-picture video index (the FVI logical model) consumed
+/// by the [`fvi_sink`]. Serialization-independent. `#[allow(dead_code)]`: the
+/// `VideoMap` accumulator is a standalone primitive staged for the side-channel
+/// (mux-while-indexing) reuse described in its module doc; the `fvi://` sink
+/// today builds `PictureRecord`s directly, so the accumulator is covered only by
+/// its own unit tests until that wiring lands.
+#[allow(dead_code)]
+pub(crate) mod videomap;
 
 pub use demux_sink::{ChaptersFmt, DelayMode, DemuxOptions, DemuxSink, Naming};
 pub use disc::DiscStream;
+pub use fvi_sink::FviSink;
 pub use m2ts::M2tsStream;
 pub use mkvstream::MkvStream;
 pub use network::NetworkStream;