ampere-av1 Phase 2.1: implement av1_set_controls body (~500 LoC)

Replaces stub av1_set_controls with full VAAPI → V4L2 stateless AV1
control translation. Four V4L2 controls batched per-frame:
  V4L2_CID_STATELESS_AV1_SEQUENCE       (sequence-level flags)
  V4L2_CID_STATELESS_AV1_FRAME          (heavy — quant, lf, cdef, lr, gm,
                                          tile_info, refs, frame flags)
  V4L2_CID_STATELESS_AV1_TILE_GROUP_ENTRY[] (DYNAMIC_ARRAY, size=MAX(N,1))
  V4L2_CID_STATELESS_AV1_FILM_GRAIN     (gated on driver_data->has_av1_film_grain)

Reference: Kwiboo/FFmpeg v4l2-request-n8.1:libavcodec/v4l2_request_av1.c
(636 LoC); same V4L2 output schema, sourced from VAAPI's
VADecPictureParameterBufferAV1 instead of FFmpeg's AV1RawSequenceHeader.

VAAPI gap notes (fields the spec needs but VAAPI doesn't expose):
  - sequence max_frame_{width,height}_minus_1 — use current frame size
  - enable_warped_motion / enable_ref_frame_mvs / enable_superres /
    enable_restoration sequence-level — conservative set-true (per-frame
    flags gate actual behavior)
  - order_hints[], reference_frame_ts[] — zero (kernel cross-refs by
    OUTPUT timestamp / surface id)
  - tile_start_col_sb[] / tile_start_row_sb[] — reconstruct via
    prefix-sum on VAAPI's width/height_in_sbs_minus_1[]
  - tile_size_bytes — set to 4 for multi-tile frames (max value), 0
    for single-tile (matches Kwiboo's conditional)
  - render_width/height — fall back to coded dimensions
  - current_frame_id / refresh_frame_flags / skip_mode_frame_idx /
    buffer_removal_time / frame_refs_short_signaling — zero
  - film_grain_params_ref_idx / update_grain — zero (only consulted in
    reuse paths; apply_grain=1 + populated arrays drive decode directly)

F1/F2/F3 risk mitigations per phase1_plan_v2:
  F1: mi_col/row_starts sentinel = 2 * ((frame_width + 7) >> 3) at
      index [tile_cols]/[tile_rows] — mirrors Kwiboo lines 238/244
  F2: superres_denom direct from VAAPI's superres_scale_denominator
      (VAAPI's encoding is the final value; no AV1_SUPERRES_DENOM_MIN
      math). Fallback to AV1_SUPERRES_NUM=8 if zero.
  F3: loop_restoration_size[] gated on USES_LR flag derived from
      y_t != 0 || cb_t != 0 || cr_t != 0 — mirrors Kwiboo lines 281-287

Plus:
  - request.h: has_av1_film_grain bool on driver_data
  - request.c: probe VIDIOC_QUERY_EXT_CTRL for FILM_GRAIN on vpu981 fd
    at VA_DRIVER_INIT (Janet v3 amendment A: init-time, not lazy)

Compile-tested on boltzmann (aarch64 native, gcc 15.2.1): clean .so,
0 errors, pre-existing GStreamer #warnings only.

Phase 3 verification on ampere is next: 208x208 smoke + film_grain
stress vector (av1-1-b8-23-film_grain-50.ivf) byte-compare libva vs
kdirect (Phase 0 proved kdirect bit-perfect).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/request.c

@@ -830,6 +830,24 @@ VAStatus VA_DRIVER_INIT_FUNC(VADriverContextP context)
 			    "vendored GStreamer parser)\n");
 	}
 
+	/*
+	 * ampere-av1 Phase 2.1: probe V4L2_CID_STATELESS_AV1_FILM_GRAIN
+	 * on the vpu981 fd. Per Janet v3 amendment, this runs at backend
+	 * init (not lazily) so any race window with concurrent device
+	 * switching can't observe an inconsistent flag.
+	 */
+	driver_data->has_av1_film_grain = false;
+	if (driver_data->video_fd_vpu981 >= 0) {
+		struct v4l2_query_ext_ctrl qec;
+		if (v4l2_query_ext_ctrl(driver_data->video_fd_vpu981,
+					V4L2_CID_STATELESS_AV1_FILM_GRAIN,
+					&qec) == 0) {
+			driver_data->has_av1_film_grain = true;
+			request_log("ampere-av1: vpu981 advertises FILM_GRAIN "
+				    "control (will include in per-frame batch)\n");
+		}
+	}
+
 	status = VA_STATUS_SUCCESS;
 	goto complete;