daedalus_v4l2: meson option gate (default true)

Adds a build-time switch so platforms that will never see a
daedalus_v4l2 kernel module (Allwinner cedrus, RK without the
shim, etc.) can opt out of the probe entry + dispatch branch.

  meson setup build                         # daedalus support on
  meson setup build-off -Ddaedalus_v4l2=false  # off

Implementation:
- meson_options.txt: new boolean `daedalus_v4l2`, default true.
- src/meson.build: when option is true, autoconfig.h gets
  `#define HAVE_DAEDALUS_V4L2 1`.
- src/request.c: known_decoder_drivers[] entry, primary-driver
  detection branch, and post-probe log line all gated by
  #ifdef HAVE_DAEDALUS_V4L2.
- src/request.h: struct daedalus fields kept UNCONDITIONAL.
  Two extra int per session and the struct layout stays stable
  across translation units regardless of option — avoids the
  ODR risk of every consumer of request.h needing to include
  autoconfig.h before request.h.

Verified on hertz: both builds compile clean.
  build/src/autoconfig.h has HAVE_DAEDALUS_V4L2; .so contains
  "daedalus_v4l2" string + log message.
  build-off/src/autoconfig.h doesn't; .so contains no daedalus
  strings at all.

Default-on build still passes vainfo end-to-end:
  vainfo: Driver version: v4l2-request
  vainfo: Supported profile and entrypoints
        VAProfileH264Main / High / ConstrainedBaseline / MultiviewHigh
        / StereoHigh : VAEntrypointVLD
        VAProfileVP9Profile0 : VAEntrypointVLD
        VAProfileAV1Profile0 : VAEntrypointVLD

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

src/request.h

@@ -105,6 +105,12 @@ struct request_data {
 	 * 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.
+	 *
+	 * Fields are unconditional (8 bytes per session) so the struct
+	 * layout is stable regardless of meson option.  The active
+	 * probe + dispatch code in request.c is gated by
+	 * HAVE_DAEDALUS_V4L2; when disabled the fields stay at their
+	 * -1 init and no codepath touches them.
 	 */
 	int video_fd_daedalus;
 	int media_fd_daedalus;