nx_visual_diff.nx

nx_visual_diff.nx -- sovereign VISUAL PARITY instrument (lib): compares two rendered screenshots (e.g. the Nishi browser vs a Chrome/Edge/Firefox oracle of the SAME page) and turns "looks the same" into NUMBERS: (1) grid-cell parity -- split the shared region into a GxG grid, compare each cell's mean color, report the matched-cell permille + the worst cells with coordinates; (2) content extent -- the last y row that differs from the page background, i.e. the page's rendered height (layout parity: if Chrome fits a page in 1500px and we take 4500px, the ratio IS the layout bug). Grid means tolerate font-rasterizer differences while catching structural drift (missing blocks, wrong colors, stretched rows). This is the S21 "same output as Chrome" benchmark's measuring stick. license_tier: ORIGINAL