fresnel-fourier iter3 Phase 6 commit B: NEW src/vp8.c + src/vp8.h

+ meson.build VP8 entries

Net-new VP8 codec dispatcher implemented against
V4L2_CID_STATELESS_VP8_FRAME (kernel UAPI <linux/v4l2-controls.h>:
1900-1958). Single batched control per frame, no init-time device-
wide menus (VP8 has no DECODE_MODE/START_CODE).

Per-frame submission: ONE VIDIOC_S_EXT_CTRLS, count=1, with full
v4l2_ctrl_vp8_frame struct (1232 bytes — corrected vs Phase 2
implicit ~400 estimate; entropy.coeff_probs[4][8][3][11] alone is
1056 bytes).

vp8_set_controls() implements 10 contract clauses per
phase4_iter3_plan.md:

  Clause 1: single-control batched submission (count=1)
  Clause 2: stack alloc + memset zero (covers all padding)
  Clause 3: width/height/version/per-frame scalars; off-by-one
            num_dct_parts = num_of_partitions - 1
  Clause 4: DPB timestamp resolution (3 refs: last/golden/alt;
            NULL surface → 0-sentinel via memset; mirrors iter1
            mpeg2.c::pic.forward_ref_ts)
  Clause 5: loop filter (6 fields + 3 flag bits; ADJ_ENABLE/
            DELTA_UPDATE/FILTER_TYPE_SIMPLE)
  Clause 6: quant base + delta derivation from VAAPI's per-segment
            absolute index matrix (subtraction recovers signed
            deltas; correct for typical content per Phase 5 S1)
  Clause 7: segment fields (segment_probs direct copy; flags
            assembled with DELTA_VALUE_MODE set unconditionally
            per FFmpeg pattern)
  Clause 8: entropy table — 3 VAAPI sources merged (Picture: y_mode +
            uv_mode + mv_probs; ProbabilityData: coeff_probs[4][8][3]
            [11] direct memcpy; IQMatrix: quant)
  Clause 9: coder state + first-partition fields + flags assembly
  Clause 10: v4l2_set_controls submission

Phase 5 review amendments incorporated:

  C1 first_part_header_bits = slice->macroblock_offset
     NOT 0 — kernel hantro_g1_vp8_dec.c:260 + rockchip_vpu2_hw_vp8_
     dec.c:372 read this field unconditionally to compute the MB-
     data DMA offset. Verified via source identity: vaapi_vp8.c:204
     and v4l2_request_vp8.c:83 use byte-identical formulas
     (8 * (input - data) - bit_count - 8); VAAPI exposes via
     slice->macroblock_offset, V4L2 names it first_part_header_bits.

  C2 first_part_size = slice->partition_size[0] +
                       ((macroblock_offset + 7) / 8)
     VAAPI's partition_size[0] is the REMAINING bytes after parsing
     (vaapi_vp8.c:209; va_dec_vp8.h:193-196). Kernel needs the
     TOTAL control partition size; recover by adding back ceil
     (macroblock_offset/8) bytes.
     Phase 3 keyframe verbatim cross-check: 21923 + 819 = 22742 ✓

  C4 (int8_t) cast (NOT (s8); s8 is kernel-internal typedef from
     <linux/types.h> not exposed to userspace; userspace UAPI
     exposes __s8 with double-underscore; portable userspace cast
     is int8_t from <stdint.h>).

  S3 assert(probability_set) — kernel hantro_vp8.c::hantro_vp8_
     prob_update reads coeff_probs unconditionally; NO default-
     table fallback. Practical risk low (FFmpeg vaapi_vp8.c always
     sends VAProbabilityBufferType per frame), but assert surfaces
     immediately if a future consumer doesn't.

Flags assembly: 6 mainline-documented bits only (KEY_FRAME, SHOW_
FRAME, MB_NO_SKIP_COEFF, SIGN_BIAS_GOLDEN, SIGN_BIAS_ALT). EXP +
bit 0x40 NOT replicated despite ffmpeg-v4l2-request-git setting
them on inter frames — kernel hantro_vp8.c only inspects KEY_FRAME
bit. SHOW_FRAME forced unconditional per Phase 3 Q4 (BBB has no
alt-ref invisible frames; documented fidelity gap).

VAAPI inverts: key_frame=0 means it IS a keyframe per VP8 spec.
Backend writes V4L2_VP8_FRAME_FLAG_KEY_FRAME iff
!picture->pic_fields.bits.key_frame.

After this commit alone: vp8.o compiles standalone; meson.build
links it into the shared library. picture.c can't dispatch yet
(commit C wires that).

Refs:
  ../fresnel-fourier/phase4_iter3_plan.md (10 contract clauses,
                                            Phase 5 amendments
                                            section)
  ../fresnel-fourier/phase5_iter3_review.md (C1, C2, C3, C4, S3
                                              all incorporated)
  ../fresnel-fourier/phase3_iter3_baseline.md (verbatim payload
                                                anchors)
  references/ffmpeg-kwiboo/libavcodec/v4l2_request_vp8.c (V4L2 ref)
  references/ffmpeg-kwiboo/libavcodec/vaapi_vp8.c (VAAPI source ref)
  references/linux-mainline/drivers/media/platform/verisilicon/
    hantro_g1_vp8_dec.c (RK3399 kernel driver — first_part_header_
    bits + first_part_size usage)

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

src/meson.build

@@ -47,7 +47,8 @@ sources = [
 	'h264_slice_header.c',
 	'request_pool.c',
 	'cap_pool.c',
-	'h265.c'
+	'h265.c',
+	'vp8.c'
 ]
 
 headers = [
@@ -70,7 +71,8 @@ headers = [
 	'h264_slice_header.h',
 	'request_pool.h',
 	'cap_pool.h',
-	'h265.h'
+	'h265.h',
+	'vp8.h'
 ]
 
 includes = [