[New]: Sub-gridding for increased performance and precision where it matters

See: https://photond.com/omnisim/features/fdtd-engine

<img width="3000" height="1012" alt="Image" src="https://github.com/user-attachments/assets/6a5a9c10-961e-4649-bdce-827ff13f01f6" />

<img width="2446" height="2000" alt="Image" src="https://github.com/user-attachments/assets/aad84b9b-e2e5-4a9c-9320-3f16a630fdee" />