Correlation: add Connection Monitor reachability evidence

[itsDNNS]

Summary

• add bucket_seconds sample-coverage metadata to the existing Connection Monitor samples response
• render Connection Monitor reachability as a separate ok / degraded / down / unknown lane without treating missing observations as healthy
• support zoom, tooltips, Connection Monitor drilldown, PNG/CSV export, responsive layouts, localization, and keyboard/screen-reader semantics
• keep speed tests as discrete observations and avoid causal or provider-responsibility claims

Checks

• 36 focused Correlation and Connection Monitor Playwright tests
• 2,921 non-E2E tests passed; 11 skipped
• desktop and mobile visual captures with no console or page errors
• node --check app/static/js/correlation.js
• git diff --check