Function ftcs_amplification 

pub fn ftcs_amplification(r: f64, k: f64, dx: f64) -> f64

Computes the von Neumann amplification factor magnitude for FTCS diffusion.

The amplification factor is |G| = |1 - 4r sin²(k dx / 2)|. Stability requires |G| ≤ 1 for all wave numbers k.