docs(guide): document the Trace Viewer #129

The trace viewer (tracing.enabled: true) writes a self-contained HTML file per failed test to target/traces/{Class}/{test}-trace.html with a clickable step timeline, screenshots, error and stack trace. It is in the changelog but has no guide page. Add docs-site/docs/guides/trace-viewer.md: enabling it, captureOnPass, the "View Trace" link in the report, how to attach it to a CI run, and a screenshot of a real trace.

Acceptance: screenshot taken from a real run; config keys verified against TraceRecorder; sidebar entry.

See CONTRIBUTING.md. Comment to claim it. Docs live in docs-site/docs/; follow an existing page as the template and run the docs site locally before opening the PR against master.