Function serial_halo_exchange_2d 

pub fn serial_halo_exchange_2d(
    locals: &mut [Array2<f64>],
    tiles: &[CartesianTile],
) -> FusionResult<()>

Serial 2D halo exchange across all tiles.

Copies owned boundary rows/columns from each tile into the halo region of its four face-neighbours. This is the serial reference implementation; the MPI version replaces this with non-blocking Isend/Irecv pairs.