Budgie's Wayland session had been broken; Plasma 6 replaces it with SDDM
still handling the Wayland greeter. Since the KDE apps in apps.nix were
already installed as GTK/MATE replacements, Budgie's exclude-package list
is no longer needed under native Plasma.
Also: OpenRGB service + udev rule, RTL-SDR/system tweaks, lmms, qemu,
qt5/qt6 qtwayland, nix flakes enabled.