rppg-skin-tone-equity: Fitzpatrick-stratified error in cross-dataset deep rPPG heart-rate estimation

Measures how much worse camera-based heart-rate estimation (rPPG) performs on darker skin, stratified by Fitzpatrick skin type on real data with ground truth, holding lighting and motion constant so skin tone is the only thing varying. Reports cross-dataset deep-model error with the caveats stated up front.