feat: add build-deb.sh with inline smoke tests T1, T2

Also fixes deps-debian.sh for Debian 13 (trixie): libqt6core6 was
renamed to libqt6core6t64; libgnubarcode-dev removed (not in Debian
13 repos, optional upstream dep).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/lib/deps-debian.sh

@@ -16,8 +16,8 @@ SETHLABELS_DEPS=(
   qt6-tools-dev qt6-tools-dev-tools
   qt6-l10n-tools
   libqt6printsupport6 libqt6svg6 libqt6widgets6 libqt6xml6 libqt6gui6
-  libqt6concurrent6 libqt6core6 libqt6test6
-  zlib1g-dev libqrencode-dev libzint-dev libgnubarcode-dev
+  libqt6concurrent6 libqt6core6t64 libqt6test6
+  zlib1g-dev libqrencode-dev libzint-dev
   file dpkg-dev fakeroot
   wget
   bats