Function diffusion_implicit_step 

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

Fully implicit (backward Euler) diffusion step.

Unconditionally stable. Solves (I - dt α D²) uⁿ⁺¹ = uⁿ.