FFmpeg + libvmaf measurement script

Cross-cutting asset B1. A dependency-free (pure standard-library) Python wrapper that measures video quality with FFmpeg + libvmaf the right way. It builds the command with the input order ([dist][ref], distorted first) and the scale-to-reference (bicubic) rule already correct, runs libvmaf for VMAF + PSNR + SSIM + MS-SSIM in one pass, parses the JSON log, and prints a report that refuses a bare 'VMAF 93' — every number is stamped with the model, the measurement resolution, and the pooling metho