🌌 Schwarzschild Ray Tracer: Black-Hole Shadow, Einstein Ring, Photon Sphere and Disc Lensing

A per-pixel ray tracer for the Schwarzschild metric: watch the star field bend into an Einstein ring around the black-hole shadow, the accretion disc wrap over the hole with Doppler beaming, and verify the critical impact parameter 3 sqrt(3) M, weak-field deflection and the ring angle against the integrated geodesics.