cd: container-native Linux packaging (deb/rpm/pacman) + AppImage

Each distro package builds inside that distro's container so declared
dependencies always resolve to native names: hand-rolled deb via
dpkg-shlibdeps + dpkg-deb, rpm via rpmbuild's auto-requires, Arch via
makepkg (non-root builder user). git/curl install before checkout
(container jobs). AppImage keeps cargo-packager on the Ubuntu runner.
The release gates on all four package jobs plus macOS/Windows.

packaging/oak.desktop

@@ -0,0 +1,8 @@
+[Desktop Entry]
+Type=Application
+Name=Oak Video Editor
+Comment=A free, open-source non-linear video editor
+Exec=oak-editor
+Icon=oak
+Categories=Video;AudioVideo;
+Terminal=false