Refactor browser metrics helpers out of runtime index

[chubes4]

Summary

• Move browser metric summaries, CDP metric normalization, bench metric promotion, and browser network/console/error serialization into browser-metrics.ts.
• Keep index.ts focused on runtime orchestration while delegating pure browser measurement helpers.
• Preserve existing browser probe/action artifact output shapes and bench metric names.

Testing

• npm run build
• npm run command-registry-smoke
• npm run package-distribution-smoke
• npm run browser-actions-artifact-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Identified and extracted the browser metrics/serialization boundary, ran targeted verification, and drafted this PR summary. Chris remains responsible for review and merge.