fix: enrich browser probe network artifacts

[chubes4]

Summary

• Capture completed Playwright request records so files/browser/network.jsonl includes request timing and transfer/body size fields when available.
• Keep browser evidence generic and extend the browser probe smoke coverage for timing and size fields.

Fixes #321

Tests

• npm run build
• npm run browser-probe-artifact-smoke
• npm run browser-actions-artifact-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the generic browser probe implementation change and focused smoke coverage; Chris remains responsible for review and validation.