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/build-deb.sh

@@ -33,10 +33,13 @@ FFMPEG_VERSION=8.1
 # epoch 2 matches Debian's stock ffmpeg (currently 7:7.1.x in trixie);
 # +rfourier suffix to avoid colliding with upstream/Debian rebuilds.
 PKGVER=2:${FFMPEG_VERSION}+rfourier+gb57fbbe
-PKGREL=5  # pkgrel=5 — H.264 IDCT 4x4 daedalus-fourier substitution; skip past
-          # an orphan -4 .deb sitting in the apt pool that made
-          # check-already-published.sh's `pool_ver ge source_full` short-
-          # circuit the previous -3 build (PR #76).  (2026-05-21)
+PKGREL=6  # pkgrel=6 — drop --enable-libxml2 to avoid runner/target libxml2
+          # SOVERSION skew (runner has libxml2 ≥ 2.14 = SONAME 16; trixie
+          # has 2.12 = SONAME 2; -5 .deb dlopens libavformat → fails on
+          # "libxml2.so.16: cannot open shared object").  Neither the
+          # daedalus-v4l2 daemon (direct AVPacket feed) nor mpv-fourier
+          # nor firefox-fourier consumers need FFmpeg's DASH demuxer.
+          # (2026-05-21)
 
 # daedalus-fourier pin — first kernel substitution in libavcodec (cycle 6
 # H.264 IDCT 4x4).  Same SHA as the daedalus-v4l2 daemon already ships
@@ -134,7 +137,6 @@ cd "$work/FFmpeg"
     --enable-libass \
     --enable-libfreetype \
     --enable-libfribidi \
-    --enable-libxml2 \
     --enable-libpulse \
     --enable-libdav1d \
     --enable-libopus \
@@ -190,7 +192,6 @@ Depends: libc6,
          libfontconfig1,
          libfreetype6,
          libfribidi0,
-         libxml2,
          libpulse0,
          libdav1d7 | libdav1d6,
          libopus0,