Function server_reachable 

pub(super) fn server_reachable() -> bool

Cheap reachability probe for the backend selector (super::detect): can we actually open an X11 connection? DISPLAY being set doesn’t guarantee a live server, so we test the real handshake (which fails fast when there’s none). Only when this succeeds do we prefer the self-centering X11/XWayland splash over the native-Wayland fallback.