fourier-debian: Path A — ffmpeg into /opt/fourier prefix

Stock Debian trixie ships FFmpeg 7.1 (libavcodec.so.61), our fork
ships FFmpeg 8.1 (libavcodec.so.62) — different SONAMEs, NOT a
drop-in for trixie's libavcodec61-consuming desktop.  Previous
Conflicts: libavcodec61, libavformat61, ... triggered apt to remove
~50 packages (kde-plasma-desktop, vlc, dolphin, ...) when a user
just wanted ffmpeg-v4l2-request-fourier installed alongside.

This commit:

1. ffmpeg-v4l2-request-fourier (pkgrel=2):
   - --prefix=/opt/fourier (instead of /usr)
   - --extra-ldexeflags / --extra-ldsoflags: -Wl,-rpath,/opt/fourier/lib
     so /opt/fourier/bin/ffmpeg finds its own libs without external help
   - Ship /etc/ld.so.conf.d/fourier.conf with /opt/fourier/lib + ldconfig
     in postinst/postrm.  dlopen-by-SONAME consumers (firefox, daedalus)
     find libavcodec.so.62 via ld.so cache without LD_LIBRARY_PATH.
   - Drop ALL Conflicts/Replaces/Provides for libav* / libpostproc /
     libsw* — no SONAME clash with stock libavcodec61, no reason to
     evict anything.
   - /usr/bin/ffmpeg-fourier + ffprobe-fourier convenience symlinks.

2. daedalus-v4l2 (pkgrel=2):
   - Depends: ffmpeg-v4l2-request-fourier (>= 2:8.1+rfourier)
     instead of stock 'ffmpeg (>= 7.1)'.  The daedalus binary was
     linked against libavcodec.so.62 at build time (CI runner had
     marfrit/ffmpeg-v4l2-request-fourier installed); at runtime it
     needs the .so.62 that only the fourier pkg provides.

Not touched:
  - libva-v4l2-request-fourier: ships only v4l2_request_drv_video.so
    at /usr/lib/<triplet>/dri/ which libva dlopens by file pattern.
    Path A would break the lookup unless every consumer launcher sets
    LIBVA_DRIVERS_PATH.  Driver name is unique; no conflict.  STAY.
  - mpv-fourier: Depends already correctly bound to fourier ffmpeg.
    Will receive libavcodec.so.62 via the ld.so.cache mechanism
    above without recipe changes.

debian/daedalus-v4l2/build-deb.sh

@@ -17,7 +17,7 @@ set -euo pipefail
 # a pixel-correct decoded frame back in ffmpeg.
 UPSTREAM_COMMIT=f55b2cdab8a8c0bc04e8c1bb1d0b6ca85e7d96d2
 PKGVER=0.1.0+r16+gf55b2cd
-PKGREL=1
+PKGREL=2  # pkgrel=2 — Depends on ffmpeg-v4l2-request-fourier (Path A SONAME .so.62)
 
 HERE=$(dirname "$(readlink -f "$0")")
 
@@ -81,7 +81,7 @@ Version: ${PKGVER}-${PKGREL}
 Section: video
 Priority: optional
 Architecture: arm64
-Depends: ffmpeg (>= 7.1), libdrm2
+Depends: ffmpeg-v4l2-request-fourier (>= 2:8.1+rfourier), libdrm2
 Recommends: daedalus-v4l2-dkms
 Maintainer: Markus Fritsche <mfritsche@reauktion.de>
 Homepage: https://git.reauktion.de/reauktion/daedalus-v4l2