Function rod_moment_of_inertia_about_center 

pub fn rod_moment_of_inertia_about_center(mass: f64, length: f64) -> Option<f64>

Computes the moment of inertia of a uniform rod about its center.

Formula: I = (1 / 12) * m * L²

Returns None when mass or length is negative, when either input is not finite, or when the computed result is not finite.