Measure every new export before it lands #417

Hold the new output to the bar the current output clears, with tests rather than good intentions.

Exports measure 96 and 97 Lighthouse performance today, 100 on accessibility, best practices and SEO. Anything shipped under this epic should be measured the same way before it lands, not after someone complains.

Done when: there is a script that runs Lighthouse against an export and fails under 95 performance or 100 accessibility, and the component kit has the same contrast check the site copy already has.