Function diffusion_explicit_step 

pub fn diffusion_explicit_step(
    u: &[f64],
    dx: f64,
    dt: f64,
    alpha: f64,
    bc_left: BoundaryCondition,
    bc_right: BoundaryCondition,
) -> Vec<f64>

Forward-time central-space (FTCS) explicit diffusion step.

Solves ∂u/∂t = α ∂²u/∂x². Stable when α dt / dx² ≤ 0.5.