ffmpeg-v4l2-request-fourier (debian): drop --enable-libxml2 (runner SONAME skew)

The Gitea debian-aarch64 runner has been upgraded past Debian trixie
and now ships libxml2 ≥ 2.14 (SONAME 16) while higgs (and any other
trixie target) still has libxml2 2.12 (SONAME 2).  -5 built cleanly,
but on higgs the daedalus-v4l2 daemon's dlopen of libavformat.so.62
fails:

    dlopen(libavformat.so.62): libxml2.so.16:
    cannot open shared object file: No such file or directory

Drop --enable-libxml2 from the Debian configure invocation; remove
the libxml2 entry from Depends; remove libxml2-dev from the CI
build-deps.  FFmpeg's libxml2-backed DASH demuxer is unused on the
Fourier fleet — daedalus-v4l2 daemon feeds AVPackets straight to
avcodec_send_packet (no demux); mpv-fourier uses ytdlp + mpv's own
stream code; firefox-fourier uses gecko-media's DASH demux.

Bumps PKGREL 5 → 6.  No source code or substitution-patch change.
Mirrors the libva trixie/runner ABI-skew workaround pattern
(marfrit-packages PR #62).

Arch PKGBUILD unaffected — Arch runner + Arch consumers both
rolling, libxml2 SONAMEs match.

After this lands, re-deploy on higgs via:
    sudo apt update && sudo apt install -y ffmpeg-v4l2-request-fourier
    sudo systemctl restart daedalus-v4l2

debian/ffmpeg-v4l2-request-fourier/debian/changelog

@@ -1,3 +1,24 @@
+ffmpeg-v4l2-request-fourier (2:8.1+rfourier+gb57fbbe-6) bookworm trixie; urgency=medium
+
+  * Drop --enable-libxml2 + libxml2 Depends — the Gitea
+    debian-aarch64 runner ships libxml2 ≥ 2.14 (SONAME 16) while
+    Debian trixie targets 2.12 (SONAME 2).  -5 built fine, then
+    failed to load on higgs trixie:
+       dlopen(libavformat.so.62): libxml2.so.16:
+       cannot open shared object file
+    Neither the daedalus-v4l2 daemon (direct AVPacket feed —
+    libavformat used only for the in-tree v4l2request hwaccel
+    glue) nor mpv-fourier (Lua + ytdlp + mpv's stream code do
+    DASH/HLS) nor firefox-fourier (gecko-media DASH demux)
+    consumes FFmpeg's libxml2-backed DASH demuxer, so dropping is
+    feature-neutral.  Mirrors the libva trixie/runner ABI-skew
+    workaround documented in PR #62.
+  * CI workflow build-deps lose libxml2-dev for the same reason.
+  * No source code change beyond configure flags + Depends.
+    Substitution stays as PRs #76/#77 landed.
+
+ -- Markus Fritsche <mfritsche@reauktion.de>  Thu, 21 May 2026 23:30:00 +0000
+
 ffmpeg-v4l2-request-fourier (2:8.1+rfourier+gb57fbbe-5) bookworm trixie; urgency=medium
 
   * pkgrel-only bump (3 → 5) to force a rebuild of the H.264 IDCT 4x4