/// Optical Metasurfaces & Flat Optics Module
///
/// Provides physics-based models for:
/// - Dielectric and plasmonic unit cells (Mie resonances, Drude model)
/// - Metalenses (hyperbolic phase, chromatic aberration, PSF)
/// - Pancharatnam-Berry / geometric-phase elements
/// - Reflectarrays and Reconfigurable Intelligent Surfaces (RIS)
///
/// All lengths in SI units (metres) unless noted otherwise.
/// Angles in radians internally; public helpers accept/return degrees where noted.
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