💎 Amethyst Optics Lab (2D): Refractive Index, Dispersion and Brilliance of a Cut Gem

A 2D ray-tracing lab for a cut gem: Snell refraction, Fresnel splitting, total internal reflection and Cauchy dispersion across seven wavelengths, with presets for amethyst, diamond and glass, a pavilion-angle sweep and four in-page numerical checks.