Boot web-vitals A/B

Lighthouse comparison of two production frontend builds. Frontend Preview uses it for an unauthenticated /login comment. There is no GMS. The comment does not replace the gzip table and does not gate the pull request.

CI serves the two dist/ trees copied by the bundle-size step (config.ci.json: Lantern, desktop, 1× CPU, 3 measured runs, 1 warm-up). Pushes to master measure the head build only. The JSON artifact is vitals-boot-pr-<n> or vitals-boot-master.

Local unit tests do not download Chrome:

cd e2e-test/perf/vitals
PUPPETEER_SKIP_DOWNLOAD=true npm ci
npm test

To compare two local builds, copy config.example.json to config.json, point builds.A.dist and builds.B.dist at the two dist/ directories, then:

npm ci
node lh-ab.mjs --config config.json --no-auth --out results/report.json --md-out results/report.md

Runs alternate A, B, A, B. A batched all-A-then-all-B loop lets host drift land on whichever build ran second. The comment is a lab delta for this runner, not a fleet p75.