Atomic distorted-wave Born approximation

[tifuchs]

Summary

• Add a native, four-channel Renaud DWBA engine for CTR calculations, exposed through the lazy SXRDCrystal.dwba state API.
  • Supports atomic bulk, UnitCell, Film, EpitaxyInterface, and PoissonSurface models.
  • Adds cached/immutable DWBA preparations, shared optical references, unique-angle field solving, vectorized glancing-angle and Vlieg geometry helpers, and observable-rich results.
  • Includes graded-interface handling, planar-reference subtraction at specular points, and DWBA-derived kinematical attenuation diagnostics.
• Move layered electric-field solving and native DWBA kernels into the C++ extension; align optical forward scattering with the structure-factor convention and hoist repeated Debye–Waller work.
• Add regression coverage for DWBA calculations, optical profiles, empty generated layers, and zero-width interface cancellation.
• Add DWBA benchmarks, design notes, API documentation, diagnostics/attenuation tutorials, and RuO2/TiO2 example notebooks.
• Improve Windows development support by recognizing Meson-built .pyd peak-search extensions.
• Centralize loading of the native CTR extension.

Breaking changes

• Replace Wavefield / solve_wavefield with LayeredElectricField / solve_electric_field.
• Core wavefield and XRR glancing-angle APIs now use radians.

Testing

• Added focused DWBA and optical-profile regression tests.

Closes

Issue #78