Playwright + axe-core a11y smoke tests. ADR-0016.

• a11y/home.spec.ts: 9-locale homepage (9 URLs)
• a11y/docs.spec.ts: 3 representative docs pages per locale (27 URLs)

Threshold: 0 serious/critical violations; <= 5 moderate per page (warning).

cd tests
npm install
npm run test:a11y:install   # one-time: downloads Chromium
npm run test:a11y

Requires website/out/ to exist (run npm run build in website/ first).

.github/workflows/governance.yml runs these on every push to main that touches website/** or tests/**.