Harden QA screenshot capture

[lissteron]

Summary

• keep the existing synchronous screenshot API backward-compatible
• capture QA screenshots from the painted canvas surface through a short-lived local media track
• support WebGL canvases without relying on drawing-buffer preservation
• resize and encode screenshots in the bridge, return structured failures, and echo request IDs
• stop only the temporary screenshot track when recording is active

Validation

• npm test (107 tests)
• npm run lint
• npm run build
• Chromium WebGL smoke with preserveDrawingBuffer disabled