feat(trace-viewer): add minimal Checkly ranged artifact support

[thebiglabasky]

Important

This draft targets main only so the Checkly delta is reviewable and can receive a Vercel Preview. Do not merge it into main.

The intended deployment base is a new checkly-next branch created at b4e7c87ce4c7f76ff1cf195d2d7d4565261ead10. Once a maintainer creates that branch, this PR should be retargeted to checkly-next.

What changed

• Keep the viewer on the latest upstream Playwright trace viewer.
• Validate Checkly's rangeStart / rangeEnd query parameters at the UI boundary.
• Carry the range as private metadata in the viewer-internal trace URI.
• Strip that metadata in the ZIP backend and issue the aggregate fetch with the correct HTTP Range header.
• Keep same-URL trace slices distinct in the service worker cache.
• Use the upstream root build command for Vercel.
• Add isolated unit and trace-viewer regression coverage.

Why

Checkly stores multiple trace ZIPs as byte ranges within one signed aggregate object. The old production fork threads the range through several upstream layers and has drifted since July 2025. This implementation limits the Checkly production integration to two upstream call sites and one isolated helper.

Impact

The trace viewer gains current upstream improvements while preserving Checkly aggregate trace loading. Ordinary Playwright trace URLs are unchanged, and malformed range metadata fails closed before fetching the aggregate.

Validation

• npx playwright test --config=tests/library/playwright.config.ts --project=chromium-library tests/library/unit/checklyTraceRange.spec.ts tests/library/trace-viewer-checkly.spec.ts — 13 passed
• npm run build — passed
• git diff --check origin/main..HEAD — passed

[vercel]

@thebiglabasky is attempting to deploy a commit to the Checkly Team on Vercel.

A member of the Team first needs to authorize it.

[thebiglabasky]

Manual Vercel preview is ready: https://playwright-trace-viewer-lxh8mhnnk-checkly.vercel.app (Checkly SSO protected). The Vercel PR check remains red only because the fork head requires GitHub authorization; the deployment itself completed successfully.