Function lumen_query_single 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn lumen_query_single(
    selector: *const c_char,
    out: *mut LumenNode,
) -> LumenStatus

Bevy single() contract: succeed only when selector matches exactly one node, writing it to *out. Zero or many matches returns LumenStatus::ErrBadArg (and sets *out to 0). Thread-safe.