daemon: dlopen Kwiboo fork's libavcodec.so.62 / libavformat.so.62 / libavutil.so.60

Switch the daemon's runtime dlopen targets from Debian-stock soname
61/61/59 (FFmpeg 7.1.3) to the Kwiboo fourier fork's soname
62/62/60 (FFmpeg 8.1) installed at the /opt/fourier prefix.

Why
---
The substitution arc tracked at daedalus-v4l2#11 needs daedalus-
fourier kernel calls woven into libavcodec's H264DSPContext NEON
init (replacing ff_h264_idct_add_neon etc. with thunks calling
daedalus_recipe_dispatch_h264_*).  We do that via patches in the
ffmpeg-v4l2-request-fourier package source — which we own, in
marfrit-packages, alongside the existing libudev-bypass and
nv15-to-p010 patches.  But that package builds the Kwiboo fork at
soname 62 / /opt/fourier.  The daemon currently dlopens soname 61
(Debian-stock + a separately-built +fourier2 patch that isn't in
marfrit-packages' source tree), so substitution patches there
wouldn't reach the daemon.

Switching to soname 62 routes the daemon through the package we
control — first step toward landing daedalus-fourier kernel
substitution into the production decode path.

Compat
------
- /opt/fourier libs are already on every host running the daemon
  (hard build-dep of ffmpeg-v4l2-request-fourier).  Firefox-fourier
  and mpv-fourier already dlopen them via the same path.
- /etc/ld.so.conf.d/fourier.conf entry resolves the new sonames
  from /opt/fourier/lib via the ld cache; dlopen-by-soname works
  without LD_LIBRARY_PATH wrappers.
- Build-side: daemon's pkg_check_modules picks up libav*.pc from
  /opt/fourier/lib/pkgconfig when PKG_CONFIG_PATH includes that
  directory (build-deb.sh follow-up will set it).
- API surface unchanged: avcodec_send_packet / receive_frame /
  AVCodecContext flags / AVFrame fields are all stable between
  FFmpeg 7.1 and 8.1.  Verified clean cross-compile on hertz.

Wire protocol unchanged.  No kmod bump.

Next step (follow-up PRs)
-------------------------
1. ffmpeg-v4l2-request-fourier patch: add 0003-daedalus-fourier-
   substitute-h264-idct4.patch that replaces ff_h264_idct_add_neon
   in libavcodec/aarch64/h264dsp_init_aarch64.c with a thunk
   calling daedalus_recipe_dispatch_h264_idct4.
2. Repeat for IDCT 8×8, deblock luma-v, qpel mc20 (one kernel per
   PR for reviewability; bench delta + decode_us delta documented
   per substitution).
3. marfrit-packages bump to pick up the new daemon + the substituted
   fourier package.

daemon/src/ffmpeg_loader.c

@@ -11,14 +11,31 @@
 #include <dlfcn.h>
 
 /*
- * SONAME versions match Debian Trixie / FFmpeg 7.1.3 today.  If
- * the system FFmpeg changes major, the daemon needs a rebuild;
- * we could add fallback paths (.so.60, .so.59, ...) but for
- * Phase 8.3 the pinned version is fine.
+ * SONAME versions match the Kwiboo ffmpeg-v4l2-request-fourier
+ * fork (FFmpeg 8.1) installed at the /opt/fourier prefix.  The
+ * fourier campaign's ld.so.conf.d/fourier.conf entry resolves
+ * these sonames from /opt/fourier/lib via the ld cache, so
+ * dlopen-by-soname works without LD_LIBRARY_PATH wrappers.
+ *
+ * Switched from Debian-stock soname 61/61/59 (FFmpeg 7.1.3) at
+ * 2026-05-21 to land daedalus-fourier kernel substitution into
+ * the production decode path via patches in the Kwiboo fork
+ * (see daedalus-v4l2#11 substitution arc): we own the fork
+ * source in marfrit-packages, so we can layer NEON-DSP
+ * substitution patches there for libavcodec/aarch64/h264dsp_init
+ * → daedalus_recipe_dispatch_* thunks.  The Debian-stock 7.1.3
+ * is built outside the marfrit-packages source tree, which
+ * would have made layering substitution patches awkward.
+ *
+ * Note: libavutil bumps soname 59 → 60 between FFmpeg 7.1 and
+ * 8.1; libavformat + libavcodec each bump 61 → 62.  The public
+ * API surface the daemon uses (avcodec_send_packet /
+ * receive_frame / AVCodecContext flags / AVFrame fields) is
+ * stable across the bump.
  */
-#define LIBAVFORMAT_SONAME	"libavformat.so.61"
-#define LIBAVCODEC_SONAME	"libavcodec.so.61"
-#define LIBAVUTIL_SONAME	"libavutil.so.59"
+#define LIBAVFORMAT_SONAME	"libavformat.so.62"
+#define LIBAVCODEC_SONAME	"libavcodec.so.62"
+#define LIBAVUTIL_SONAME	"libavutil.so.60"
 
 /*
  * Resolve a symbol from a dlopen'd handle.  Logs the failure