Function laplace_solve 

pub fn laplace_solve(
    grid: &mut Vec<f32>,
    boundary_mask: &[bool],
    width: usize,
    height: usize,
    iterations: usize,
)

Solve Laplace’s equation ∇²φ = 0 on a grid using Jacobi/SOR iteration. boundary_mask[i] = true means the cell is a fixed boundary.