No-reference color-artifact detector

A small, self-contained Python script that inspects a single frame (or folder of frames) for color artifacts WITHOUT a reference, using three blind checks on the color channels: (1) chroma detail ratio — the L2 (squared-gradient) edge energy of the color channels relative to the brightness channel, which drops when subsampling softens color edges (mean/L1 gradient is conserved by subsampling and cannot see it, which is why the script uses squared-gradient energy); (2) chroma edge width in pixels