WGS-84 geodetic ↔ ECEF conversion, delegated to the Rust NIF so the transform matches the solver's geometry exactly (and so callers need no separate geodesy dependency).

Constants: semi-major axis a = 6_378_137.0 m, flattening f = 1 / 298.257223563, first eccentricity² e² = f(2 − f). The forward transform is exact closed form; the inverse uses Bowring's method.