Ray Tracing in Bend 2

A Bend 2 implementation of the Ray Tracing in a Weekend scene, ported from the author's Nim version. It renders a scene of spheres to TGA, with optional PPM output and configurable sample counts.

Rendering splits interleaved image rows across a balanced tree of parallel calls. The included laws and proofs cover row ordering, output dimensions, and equivalence between parallel and sequential row assembly. The project also documents the limits of those proofs: floating-point shading correctness is not established by them.