Function start_cu_server 

pub fn start_cu_server(bind: &str, token: Option<&str>) -> Result<(), String>

Start the CU server on bind, comma-separated entries: a bare port listens on the loopback addresses only, host:port names the address to listen on (0.0.0.0:port,[::]:port accepts every interface), requiring token, else PIXELFLUX_CU_TOKEN, of every request. Guarded so that exactly one server binds per process no matter how many call sites (module import, Wayland compositor init, the selkies setting) race to spawn it; backend selection stays per-request, so a server bound at import serves a compositor that only starts later.