uzor-proof-harness 1.5.1

Dev-only multi-backend (tiny-skia / vello-cpu / vello-gpu / urx-cpu / urx-gpu) headless proof-render harness: drives one RenderContext-driven draw closure through every available rasterizer backend (three CPU legs always, two GPU legs gracefully skipped when no adapter is available), reports pairwise pixel-diff fractions, and writes a labelled side-by-side composite PNG. Promoted out of uzor-examples/src/parity_harness.rs (URX Wave 6) so any publish = false engine crate (uzor-figures first) can reach it as a dev-dependency without adding a runtime dependency to its own [dependencies].