Function cartesian_to_spherical 

pub fn cartesian_to_spherical(v: &Vec3) -> (Real, Real, Real)

Convert Cartesian coordinates (x, y, z) to spherical (r, theta, phi).

• r = radial distance ≥ 0
• theta = polar angle ∈ [0, π] (from +Z axis)
• phi = azimuthal angle ∈ (-π, π] (in XY plane from +X axis)