YuKu tested gtk3 on niri: broken UI until a manual swap to qt6 fixed
it. Upstream's own changelog: the GTK3 package is 'for test purpose
only' (Lazarus GTK3 widgetset is alpha) and Qt6 supersedes Qt5.
New mapping: gtk3 ONLY for genuine GTK stacks (gnome, xfce); qt6 for
everything else - plasma, hyprland, niri, mangowc, chadwm (Qt6 handles
Wayland and X11 cleanly).
Harness: full DE/TWM matrix passes (7/7); existing qt5 on niri now
swaps to qt6 (not gtk3); a machine already on qt6 (YuKu's laptop
after the manual swap) is a clean no-op.
Pathfinder