test(rate): add cross-browser and PyQt visual gates

[d-sorgcodexagent]

Summary

• extend production variation E2E to Chromium, Firefox, and WebKit on the ephemeral PR lane
• add deterministic PyQt rendered-interaction diagnostics at 100% and 150% scale for mesh toggle, camera, zoom/auto-fit, legend visibility, and overlap
• exercise real localized shoulder/wrist+yaw production Worker results with nonzero full-rank confidence surfaces and keyboard controls
• keep trusted fleet execution explicitly Chromium-only and main-push-only while widening PR triggers to every scientific/UI authority under test

Verification

• 874 React tests
• 9/9 all-engine production Playwright scenarios
• 7/7 trusted Chromium project scenarios
• 20 focused PyQt/workflow/validator tests
• actionlint, 67-workflow validation, runner trust guard, MyPy/Ruff/TS/ESLint/Vite/docs/size/diff
• independent exact-head GO review

Stack

Draft child of #4417; this branch directly inherits its exact published head, so no synthetic merge was created. Screenshots are diagnostic artifacts, not approved pixel goldens. Formal screen-reader/axe certification, mobile cross-engine coverage, prescribed Worker transport, and remaining #4142 work remain open.