fresnel-fourier iter1 Phase 6 commit B: rewrite mpeg2.c against new V4L2 stateless API

Rewrites src/mpeg2.c to submit MPEG-2 control payload via the new
split V4L2_CID_STATELESS_MPEG2_{SEQUENCE,PICTURE,QUANTISATION}
controls (mainline kernel <linux/v4l2-controls.h>:1985-2105),
replacing the staging-era V4L2_CID_MPEG_VIDEO_MPEG2_{SLICE_PARAMS,
QUANTIZATION} combined-struct API that the kernel removed.

Per-frame submission: one batched VIDIOC_S_EXT_CTRLS, count=3,
ctrl_class=V4L2_CTRL_CLASS_CODEC_STATELESS (0xf010000), with the
three controls in order:
  - id=0xa409dc (SEQUENCE)      size=12  bytes
  - id=0xa409dd (PICTURE)       size=32  bytes
  - id=0xa409de (QUANTISATION)  size=256 bytes

Matches FFmpeg libavcodec/v4l2_request_mpeg2.c:130-155 reference
implementation. Verified empirically against fresnel-fourier
Phase 0 cross-validator anchor (bit-for-bit byte equivalence on
SEQUENCE first-row + QUANTISATION 256 bytes).

Six structural changes from old to new API:

  1. Slice header parsing moved to kernel: bit_size,
     data_bit_offset, quantiser_scale_code GONE from new structs.
  2. Reference timestamps moved from slice to picture:
     forward_ref_ts/backward_ref_ts now in
     v4l2_ctrl_mpeg2_picture (offsets 0/8).
  3. Boolean fields collapsed into picture.flags bitmask
     (TOP_FIELD_FIRST 0x01 .. PROGRESSIVE 0x80, 8 bits total).
  4. progressive_sequence collapsed into sequence.flags &
     V4L2_MPEG2_SEQ_FLAG_PROGRESSIVE.
  5. PICTURE_CODING_TYPE renamed to PIC_CODING_TYPE (values same).
  6. Quantisation load_* flags removed; matrices always present;
     British spelling — quantiSation not quantiZation.

Behavioral correction (from old code, was a latent bug):
  Old src/mpeg2.c:104-118 self-referenced surface_object timestamp
  when the VAAPI ref picture was VA_INVALID_ID. New code sets the
  ref_ts to 0, matching kernel doc 0-as-sentinel convention
  (verified Phase 3 Baseline C: I-frame has both ts == 0; FFmpeg
  v4l2_request_mpeg2.c:98-108 same convention).

Quantisation matrix order: zigzag scanning order per kernel doc
v4l2-controls.h:2076. VAAPI VAIQMatrixBufferMPEG2 stores in
zigzag order (per VAAPI spec). Direct memcpy works; no
permutation in libva backend. Kernel hantro_mpeg2.c::
hantro_mpeg2_dec_copy_qtable applies zigzag-to-raster permutation
when copying to the hardware quantisation table.

Default matrices (when iqmatrix_set==false): MPEG-2 spec defaults
per ISO/IEC 13818-2 Table 7-3. The mpeg2_default_intra_matrix
constant was transcribed from fresnel-fourier Phase 3 Baseline C
QUANTISATION verbatim payload bytes 0..63 (256-byte capture from
ffmpeg-v4l2request decode of bbb_720p10s_mpeg2.ts), per
phase5_iter1_review.md S3 amendment that flagged spec-recall as
unreliable. non_intra and chroma_non_intra are 16s per spec
(verified Baseline C bytes 64..127, 192..255). chroma_intra is
copy of intra (Baseline C bytes 128..191, verified identical).

Submission shape: one batched v4l2_set_controls call with all
three v4l2_ext_control entries, matching iter6/7/8 H.264 pattern
at src/h264.c:986. Bound to surface_object->request_fd (the
per-OUTPUT-slot permanent request_fd from iter6 binding).

Behavioral details:
  - sequence.vbv_buffer_size = surface_object->source_size, where
    source_size is set in picture.c:276 from request_pool slot->size,
    which is the V4L2-negotiated sizeimage from VIDIOC_QUERYBUF.
    Matches FFmpeg controls->pic.output->size.
  - sequence.profile_and_level_indication = 0; not exposed by
    VAAPI VAPictureParameterBufferMPEG2.
  - sequence.chroma_format = 1 (4:2:0) hardcoded; campaign codec
    scope is 4:2:0.
  - progressive_frame proxies for progressive_sequence; same bit
    for typical streams.

Phase 6 smoke test (post Commit A + Commit B):
  - vainfo enumerates VAProfileMPEG2Simple + VAProfileMPEG2Main
    on hantro bind. (Phase 1 criterion 1)
  - libva trace: vaCreateConfig(VAProfileMPEG2Main) =
    VA_STATUS_SUCCESS. (Phase 1 criterion 2)
  - ffmpeg -hwaccel vaapi exits 0 with no Failed-to-create-
    decode-configuration. (Phase 1 criterion 3 adjusted)
  - mpv --hwdec=vaapi --vo=image at +02s seek: 2 distinct
    frames with hashes byte-identical to SW reference:
      HW frame 1: 6e7873030dbf0403c67f35dd106ebef3c7909a0fd12433b82ad758e7fee9f092
      SW frame 1: 6e7873030dbf0403c67f35dd106ebef3c7909a0fd12433b82ad758e7fee9f092
      HW frame 2: ccc7ce08810d4a96e9ba7a19f4f95bbf6cc861bda9337604b5c668ad52bef7de
      SW frame 2: ccc7ce08810d4a96e9ba7a19f4f95bbf6cc861bda9337604b5c668ad52bef7de
    (Phase 1 criterion 4 — DMA-BUF GL import path; cache-coherency-safe)
  - T4 H.264 reference hashes still match (criterion 5; verified
    Phase 3 Baseline D earlier).

Cache-stale class observation (out-of-scope iter1 work item):
  ffmpeg -hwaccel vaapi -hwaccel_output_format vaapi + hwdownload
  pipeline produces all-zero NV12 for MPEG-2 (same iter1 patch-0011
  cache-coherency bug class observed for H.264 in fresnel-fourier
  T4). Kernel + HW decode is correct (verified via ffmpeg
  -hwaccel v4l2request -hwaccel_output_format drm_prime + hwdownload
  which produces correct non-zero pixels matching SW reference).
  Bug is in libva backend vaDeriveImage path; Phase 4 cross-
  cutting work to add VIDIOC_EXPBUF + DMA_BUF_IOCTL_SYNC support.
  Not blocking iter1 — DMA-BUF GL import path (mpv --vo=image) is
  cache-coherency-safe and gives bit-exact pixels.

Auxiliary EINVAL noise (out-of-scope iter1 work item):
  src/context.c:142-155 unconditionally sets H.264 device-wide
  controls (V4L2_CID_STATELESS_H264_DECODE_MODE,
  _START_CODE) on every CreateContext, regardless of profile.
  EINVALs on hantro-vpu-dec (no H.264 controls there). Intentional
  best-effort behavior — return value cast to (void) and discarded
  at line 153. The error message "Unable to set control(s):
  Invalid argument" is logged from src/v4l2.c:484 but doesn't
  propagate as a backend error. Stays as documented auxiliary
  noise.

Drop #include <mpeg2-ctrls.h> from src/config.c:37 and src/mpeg2.c
(formerly line 38). The kernel UAPI for MPEG-2 stateless control
IDs comes from <linux/v4l2-controls.h>, pulled transitively via
<linux/videodev2.h> (and explicitly from src/mpeg2.c after this
rewrite). The fork local include/mpeg2-ctrls.h header is deleted
in commit C; this commit removes the last includes of it.
src/config.c:38 still includes <hevc-ctrls.h> — left untouched per
phase5_iter1_review.md Nit 6 (lower-risk path; HEVC iteration
deletes its header).

Refs:
  ../fresnel-fourier/phase4_iter1_plan.md (contract clauses 1-6,
                                           File 2 patch shape)
  ../fresnel-fourier/phase5_iter1_review.md (S3, Q4, Q5 amendments)
  ../fresnel-fourier/phase0_evidence/2026-05-07/iter1_phase3/
    baseline_C_xvalidator/ffmpeg.stdout (cross-validator anchor)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/config.c

@@ -34,7 +34,6 @@
 
 #include <linux/videodev2.h>
 
-#include <mpeg2-ctrls.h>
 #include <hevc-ctrls.h>
 
 #include "utils.h"