From-scratch MS-SSIM calculator

A dependency-light Python script that computes MS-SSIM between two image or frame files from first principles. It prints the per-scale contrast x structure (cs) terms across all five scales, the luminance term at the coarsest scale, the calibrated weights, and the final weighted product, alongside the single-scale SSIM for comparison. Includes a --demo mode (fine-grain-noise case), an explicit minimum-size fallback that renormalizes the weights, and the equivalent FFmpeg/libvmaf float_ms_ssim co