pub fn pseudo_potential(x: f64, y: f64, z: f64, mu: MassRatio) -> f64
Pseudo-potential Ω(x, y, z) in the synodic frame.
Ω = ½(x² + y²) + (1-μ)/r₁ + μ/r₂
where r₁, r₂ are distances to the two primaries.