av1: populate V4L2_CID_STATELESS_AV1_SEQUENCE in codec_set_controls

Implements the libva-side portion of issue #11 — replaces PR #10's
no-op AV1 dispatch with a real av1_set_controls that maps VAAPI's
VADecPictureParameterBufferAV1.seq_info_fields + scalar fields onto
struct v4l2_ctrl_av1_sequence (the kernel uAPI control declared at
linux/v4l2-controls.h:2891-2919).

Daemon-track context (issue #11 daemon side, operator-owned):
ffmpeg-vaapi splits the AV1 bitstream client-side and strips the
OBU_SEQUENCE_HEADER before delivery; the V4L2 OUTPUT buffer contains
only OBU_FRAME_HEADER + OBU_TILE_GROUP.  libdav1d in the daedalus
daemon cannot parse this — it expects a complete OBU stream.  The
daemon side has to synthesise OBU_SEQUENCE_HEADER from the SEQUENCE
ctrl and prepend it to the slice bitstream.  This libva-side change
just makes the SEQUENCE ctrl populated and queued via S_EXT_CTRLS;
the daemon track is the consumer.

Three small touch points beyond the new src/av1.{c,h}:

  - src/surface.h: add an av1 leaf to surface->params holding
    VADecPictureParameterBufferAV1.  Slice params intentionally
    absent — the daedalus daemon consumes the slice OBU bytes
    directly from the OUTPUT buffer; no per-tile-group struct →
    OBU re-synthesis required from libva today.
  - src/picture.c: copy the picture-param buffer into the new leaf
    in RenderPicture, mirror of the per-codec memcpy pattern, plus
    call av1_set_controls from codec_set_controls (replacing the
    no-op).
  - src/meson.build: register src/av1.c.

Sequence-field mapping covers everything VAAPI exposes at the
sequence level (12 of 18 V4L2_AV1_SEQUENCE_FLAG_* bits + the four
scalars).  Bits VAAPI doesn't carry at the sequence level
(WARPED_MOTION, REF_FRAME_MVS, SUPERRES, RESTORATION,
SEPARATE_UV_DELTA_Q) stay clear; per-frame consumers (libdav1d via
the daemon, vpu981 via the hardware path) read those from the
OBU_FRAME_HEADER that is already in the slice buffer anyway.  See
feedback memory `feedback_vaapi_blind_to_some_hevc_sps_fields` for
the precedent.

Build verified on higgs (Debian 13 trixie, gcc 14.2.0, libva 2.22.0,
linux uAPI v4l2-controls.h sizeof(struct v4l2_ctrl_av1_sequence)==12):
clean meson + ninja link of v4l2_request_drv_video.so, vainfo
enumerates VAProfileAV1Profile0 via daedalus_v4l2 slot, av1_set_controls
symbol present.

Out of scope on this PR (operator-track, issue #11 follow-up):
  - daedalus-v4l2 kernel module wire-protocol extension (daedalus_
    collect_av1_meta + AV1 ctrl request_setup).
  - daedalus daemon OBU synthesiser (~400 LoC AV1 OBU encoder in
    daemon/src/av1_obu_synth.{c,h}).

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

src/av1.h

@@ -0,0 +1,39 @@
+/*
+ * Copyright (C) 2026 Markus Fritsche <fritsche.markus@gmail.com>
+ *
+ * AV1 codec dispatcher — populates V4L2_CID_STATELESS_AV1_SEQUENCE
+ * (struct v4l2_ctrl_av1_sequence) from VAAPI's VADecPictureParameterBufferAV1.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sub license, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the
+ * next paragraph) shall be included in all copies or substantial portions
+ * of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+ * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT.
+ * IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE FOR ANY CLAIM,
+ * DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+ * OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR
+ * THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#ifndef _AV1_H_
+#define _AV1_H_
+
+#include "context.h"
+#include "request.h"
+#include "surface.h"
+
+int av1_set_controls(struct request_data *driver_data,
+		     struct object_context *context,
+		     struct object_surface *surface);
+
+#endif /* _AV1_H_ */