Add Angular Spectrum wave propagation module

[gustavu92]

Draft – Angular Spectrum Wave Propagation (ASM)

This is an early draft PR to get feedback before continuing the implementation.

It is not the final version. I would like your suggestions on API design and tensor layout before refining and expanding the module.

What is included

• New module: optiland.wavepropagation
  • BaseWavePropagator
  • AngularSpectrumPropagator (ASM implementation)
• Evanescent handling modes: "clamp" and "decay"
• Padding support for propagation
• Backend extensions:
  • NumPy: fftfreq, fft2, ifft2, meshgrid, pad, clamp, broadcast_to, zeros
  • Torch: clamp
• A development tutorial notebook demonstrating:
  • Field computation at multiple z planes
  • Stacked output (S, F, W, N, N)
  • Intensity visualization along propagation

The notebook is for clarity and experimentation. It can be simplified or adjusted before moving into docs/examples.

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Main Design Question

I am unsure about the expected output layout of compute_field.

Currently I support:

• (W, N, N) for a single field
• (F, W, N, N) for multiple fields

In the tutorial, I standardize everything to (S, F, W, N, N) when stacking over propagation distance.

I am not fully convinced this is the best convention.

Questions:

• What should be the canonical field tensor layout?
• Should field or wavelength be the leading dimension?
• Is (F, W, N, N) aligned with the rest of Optiland design?

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I would appreciate early feedback on:

• API structure
• Tensor layout convention
• Integration approach with Optic
• Any architectural concerns before I continue development

Once the structure is validated, I will clean up the implementation and prepare a final PR.