Function gpu_jacobi_pressure_solve 

pub fn gpu_jacobi_pressure_solve(
    grid: &mut MacGrid,
    rho: f64,
    dt: f64,
    iterations: usize,
)

GPU Jacobi pressure solver on a MAC grid.

Identical algorithm to MacGrid::jacobi_pressure_solve but expressed as a function that could be launched as a compute shader (one thread per cell). The indirection through a separate function makes the GPU dispatch boundary explicit.