Content-type quality benchmark tool

A dependency-free (pure standard-library) Python tool that loads the per-content rate-quality dataset and computes BD-rate (HEVC, AV1 over H.264 and AV1 over HEVC) for every content type, the spread across content types, the content-complexity table (anchor bitrate at VMAF 93 with ITU-T P.910 SI/TI), and a CATALOGUE-WEIGHTED saving for any content mix via --mix (e.g. --mix animation=60,screen=20,film=20). Reproduces this article's per-content table and worked mixes exactly and matches article 7.