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>

meson_options.txt

@@ -4,3 +4,14 @@ option(
     value : '',
     description: 'Path to sanitized Linux Kernel headers'
 )
+
+option(
+    'daedalus_v4l2',
+    type : 'boolean',
+    value : true,
+    description: 'Enable probe + dispatch for the out-of-tree daedalus_v4l2 ' +
+                 'stateless decoder shim (Pi 5 / CM5 daemon-backed VP9/AV1/H264). ' +
+                 'Default true; disable on platforms where the daedalus_v4l2 ' +
+                 'kernel module will never be present to slim the probe array.'
+)
+