daedalus_v4l2: add to known_decoder_drivers + multi-device-probe slot

Phase 8.10 of the daedalus-v4l2 sibling campaign — out-of-tree
V4L2 stateless decoder shim that forwards bitstream to a
userspace daemon (FFmpeg-software decode for VP9 / AV1 / H.264;
pixels back via dmabuf into the CAPTURE buffer).

Adds the same iter40-shaped wiring as rpi-hevc-dec:
- known_decoder_drivers[] entry "daedalus_v4l2"
- video_fd_daedalus + media_fd_daedalus slots in driver_data
- -1 init alongside the other multi-device slots
- primary-driver detection branch in the auto-probe block
- post-probe log line for symmetry with iter40

No per-profile dispatch changes needed — daedalus_v4l2 advertises
the standard V4L2_PIX_FMT_{VP9_FRAME,AV1_FRAME,H264_SLICE}
OUTPUT fourccs the fork's existing per-driver paths already
handle.

Verified on hertz (Pi 5 / CM5, 6.12.75+rpt-rpi-2712) with the
daedalus_v4l2 module loaded:

  LIBVA_DRIVER_NAME=v4l2_request \
  LIBVA_V4L2_REQUEST_VIDEO_PATH=/dev/video0 \
  LIBVA_V4L2_REQUEST_MEDIA_PATH=/dev/media3 \
  vainfo --display drm --device /dev/dri/renderD128

  v4l2-request: opened daedalus_v4l2 at video_fd=... media_fd=... (Pi 5 daemon-backed VP9/AV1/H264)
  vainfo: Driver version: v4l2-request
  vainfo: Supported profile and entrypoints
        VAProfileH264Main               : VAEntrypointVLD
        VAProfileH264High               : VAEntrypointVLD
        VAProfileH264ConstrainedBaseline: VAEntrypointVLD
        VAProfileH264MultiviewHigh      : VAEntrypointVLD
        VAProfileH264StereoHigh         : VAEntrypointVLD
        VAProfileVP9Profile0            : VAEntrypointVLD
        VAProfileAV1Profile0            : VAEntrypointVLD

Without the env override the auto-probe still picks rpi-hevc-dec
first (it's earlier in known_decoder_drivers[]); on the standalone
daedalus_v4l2 path the daemon-backed decode is what answers
S_FMT/QBUF/DQBUF. On a mixed-driver Pi 5 box where both modules
are loaded, HEVC continues to route through rpi-hevc-dec via the
existing 'p' override; VP9/AV1/H264 would prefer daedalus_v4l2
since rpi-hevc-dec is HEVC-only.

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

src/request.h

@@ -96,6 +96,18 @@ struct request_data {
 	 */
 	int video_fd_rpi_hevc_dec;
 	int media_fd_rpi_hevc_dec;
+	/*
+	 * phase 8.10: fifth multi-device-probe slot for daedalus_v4l2 — the
+	 * out-of-tree V4L2 stateless decoder shim that forwards bitstream
+	 * to a userspace daemon (daedalus-v4l2 sibling repo). Daemon does
+	 * FFmpeg-software decode for VP9 / AV1 / H.264 and ships pixels
+	 * back via dmabuf into the CAPTURE buffer.  Picked up via the
+	 * same media-controller probe + known_decoder_drivers[] entry
+	 * pattern as iter40 rpi-hevc-dec.  Stays -1 on hosts without the
+	 * daedalus module loaded; HEVC routes to rpi-hevc-dec as before.
+	 */
+	int video_fd_daedalus;
+	int media_fd_daedalus;
 	/*
 	 * ampere-av1-enablement Phase 2: fourth multi-device-probe slot
 	 * for vpu981 (RK3588's dedicated AV1 hantro instance, kernel