Critical request chain visualizer

Everything runs in this tab; the file is never uploaded and no URL is fetched. Chain cost is the end of the last request minus the start of the document request, taken from the capture's own timestamps, so it reflects that one load and not an average. Script async and defer are not visible in HAR, so parser-initiated scripts are reported as blocking candidates rather than certainties. HTTP/2 and HTTP/3 multiplexing, service workers, and priority hints change what a chain costs in practice; treat the fix list as a starting point for a DevTools or Lighthouse pass, not as the whole story.