Function solve_2x2 

pub fn solve_2x2(
    a11: f64,
    a12: f64,
    b1: f64,
    a21: f64,
    a22: f64,
    b2: f64,
) -> Option<(f64, f64)>

Solves a 2x2 linear system with Cramer’s rule.

Returns None when the determinant is zero, non-finite, or when the computed solution is not finite.