/// Diffractive optics: gratings, holographic DOEs, Fresnel/diffractive lenses,
/// scalar diffraction propagation (Rayleigh-Sommerfeld, Fresnel, Fraunhofer),
/// and grating spectrometer design.
///
/// Physical conventions used throughout:
/// - Wavelengths in nm
/// - Distances in μm unless noted mm
/// - Angles in radians unless noted deg
/// - Refractive indices dimensionless
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