test: add confidence suite, metrics docs, and analyzer updates

Why:
Analyzer output is no longer capped, so tests must reflect full
results. Confidence scoring needs full boundary coverage. The HTML
report also requires a metrics reference for users.

What:
- Update analyzer tests to expect uncapped counts
- Add confidence.test.ts for score and label coverage
- Add html-report-metrics.md describing all metrics

Author: doguabaris