Responsive image srcset builder

The download table simulates the standard browser choice: the smallest file whose width covers the layout slot times the pixel density, or the largest file when none is wide enough. Real browsers can deviate on slow connections and when a candidate is already in cache, and only min-width and max-width conditions with px, vw, em, and rem lengths are simulated; calc(), container queries, and sizes="auto" are refused rather than guessed. em and rem slots assume a 16 px root font size. The tool generates references to files like photo-800.webp; you still have to create those files at those widths.