fresnel-fourier iter4 Phase 6 commit B: NEW src/vp9.c + src/vp9.h + meson.build + context.h (vp9_lf) + surface.h (params.vp9)

VP9 codec dispatcher implementing 12 contract clauses against
V4L2_CID_STATELESS_VP9_FRAME (0xa40a2c) +
V4L2_CID_STATELESS_VP9_COMPRESSED_HDR (0xa40a2d). 2 batched
controls per frame; rkvdec on RK3399 mandatorily requires both
per drivers/staging/media/rkvdec/rkvdec-vp9.c::rkvdec_vp9_run_preamble:752.

Implementation:
- ~80 LOC VPX range coder (vp9_rac_*) — minimal port of FFmpeg
  vpx_rac.[ch] + vp89_rac.h. Stateless static helpers.
- inv_map_table[255] + read_prob_delta — verbatim copy from
  v4l2_request_vp9.c:44-97.
- vp9_parse_uncompressed_header_lf_quant — partial parse for the
  fields VAAPI doesn't expose: lf_delta_enabled / lf_delta_update /
  lf_ref_delta[4] / lf_mode_delta[2] / base_q_idx /
  delta_q_y_dc / delta_q_uv_dc / delta_q_uv_ac. ~120 LOC.
- vp9_fill_compressed_hdr — port of FFmpeg fill_compressed_hdr
  with Phase 5 C3 out_reference_mode parameter. ~140 LOC.
- vp9_set_controls — orchestrates Clauses 1+2+4+5+7+10+11+12.
  ~120 LOC.

Phase 5 amendments incorporated in code:
- C1: frame.interpolation_filter = direct from VAAPI's
  mcomp_filter_type (NO XOR; vaapi_vp9.c:62 already applied it
  before storing into VAAPI's mcomp_filter_type).
- C2: persistent vp9_lf state added to object_context (in
  context.h). Initialized to VP9 spec defaults
  {1,0,-1,-1,0,0} on keyframe / intra_only / error_resilient.
  Updated only when parser sees lf_delta.update=1. Always
  copied to kernel control.
- C3: vp9_fill_compressed_hdr takes uint8_t *out_reference_mode;
  threaded through call site. allowcompinter derived from VAAPI
  sign-bias bits.

Phase 5 S4: uv_mode memcpy from FFmpeg's fill_compressed_hdr
omitted — rkvdec reads uv_mode from kernel's persistent
probability_tables, NOT from prob_updates ctrl.

Clause 3 compile-time _Static_assert on struct sizes (168/2040)
matches Phase 3 empirical baseline; UAPI shifts will fail loudly.

surface.h: extends params union with vp9 { picture, slice }.
context.h: adds vp9_lf { ref_deltas[4], mode_deltas[2], initialized }.
meson.build: adds vp9.c + vp9.h.

Build: clean on fresnel (linux-fresnel-fourier 7.0-1, libva 1.23).
Runtime: not yet wired in picture.c — next commit.

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

src/context.h

@@ -27,6 +27,9 @@
 #ifndef _CONTEXT_H_
 #define _CONTEXT_H_
 
+#include <stdbool.h>
+#include <stdint.h>
+
 #include <va/va_backend.h>
 
 #include "object_heap.h"
@@ -51,6 +54,17 @@ struct object_context {
 	/* H264 only */
 	struct h264_dpb dpb;
 	bool h264_start_code;
+
+	/* fresnel-fourier iter4: VP9 loop-filter delta state, persisted across
+	 * frames per kernel UAPI <linux/v4l2-controls.h>:2578 ("If this syntax
+	 * element is not present in the bitstream, users should pass its last
+	 * value.") and VP9 spec defaults from FFmpeg vp9.c:666-671. Reset on
+	 * keyframe / error-resilient / intra-only via vp9_lf.initialized=false. */
+	struct {
+		int8_t ref_deltas[4];
+		int8_t mode_deltas[2];
+		bool initialized;
+	} vp9_lf;
 };
 
 VAStatus RequestCreateContext(VADriverContextP context, VAConfigID config_id,