fourier-debian: fix mpv Conflicts: + daedalus ffmpeg conflict

Two deterministic CI failures from run 115 (PR #34 wiring):

mpv-fourier-debian (task 566): dpkg-deb refuses `Conflicts: pkg1 |
pkg2` — Debian policy doesn't allow alternatives in Conflicts.
Split into separate entries.

daedalus-v4l2-debian (task 563): pacman -Syu pulled stock arch
ffmpeg which conflicts with the already-installed
ffmpeg-v4l2-request-fourier left behind by the earlier
mpv-fourier-aarch64 job (it configures [marfrit] + pre-installs
the fourier ffmpeg).  Drop 'ffmpeg' from the pacman line; mirror
the [marfrit]+pre-install step from mpv-fourier-debian.  Daedalus
only needs libavcodec/libavformat headers which the fourier
package supplies.

.gitea/workflows/build.yml

@@ -978,13 +978,37 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: install build-deps
+      - name: install build-deps (sans ffmpeg — see [marfrit] step)
         run: |
           set -e
           retry() { for i in 1 2 3; do "$@" && return 0; rc=$?; echo "retry $i (exit=$rc)" >&2; sleep $((i*5)); done; return 1; }
+          # Do NOT pull stock 'ffmpeg' here: the arch-aarch64 runner has
+          # ffmpeg-v4l2-request-fourier pre-installed from the mpv-aarch64
+          # job (configured via [marfrit]), and pacman -S ffmpeg would
+          # conflict on the libav* drop-in.  Daedalus build only needs
+          # libavcodec/libavformat headers, which the fourier package
+          # already supplies.  Keep cmake/ninja/pkgconf/libdrm here; the
+          # ffmpeg-dev equivalent comes via the next step.
           retry pacman -Syu --noconfirm --needed \
             dpkg openssh rsync curl base-devel git cmake ninja pkgconf \
-            ffmpeg libdrm
+            libdrm
+
+      - name: ensure ffmpeg-v4l2-request-fourier installed (link-time ABI source)
+        run: |
+          set -e
+          # Idempotent: pre-install the marfrit fourier ffmpeg so cmake
+          # finds libavcodec / libavformat / libavutil headers + .so's.
+          # Mirrors mpv-fourier-debian's [marfrit] step.
+          curl -sLo /tmp/marfrit.gpg https://packages.reauktion.de/marfrit.gpg
+          pacman-key --add /tmp/marfrit.gpg
+          pacman-key --lsign-key 92D5E96D8F63C75E4116AA1FF5C8C4603D0D250C
+          rm -f /tmp/marfrit.gpg
+          if ! grep -q '^\[marfrit\]' /etc/pacman.conf; then
+            printf '\n[marfrit]\nServer = https://packages.reauktion.de/arch/$arch\nSigLevel = Required\n' >> /etc/pacman.conf
+          fi
+          pacman -Sy --noconfirm
+          rm -f /var/cache/pacman/pkg/ffmpeg-v4l2-request-fourier-*-aarch64.pkg.tar.*
+          printf 'y\ny\ny\n' | pacman -S --needed marfrit/ffmpeg-v4l2-request-fourier
 
       - name: install hertz deploy ssh key
         env: