Function diffusion_crank_nicolson_step 

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

Crank-Nicolson implicit diffusion step.

Solves ∂u/∂t = α ∂²u/∂x² using the unconditionally stable C-N scheme. Interior points are advanced; boundary values are enforced via bc_left / bc_right.