Function fdtd_wave_step_cpu 

pub fn fdtd_wave_step_cpu(
    pressure: &[f32],
    pressure_prev: &mut [f32],
    c2: f32,
    damping: f32,
    idx: usize,
    buffer_width: usize,
)

FDTD wave equation kernel for 16x16 tiles with 1-cell halo.

This is a pure Rust implementation of the FDTD step that can be used for CPU fallback and verification.