Require label coverage for strict corpus validation

[heznpc]

Summary

• add label coverage metrics to corpus validation output
• make strict release validation fail when any report findings remain unreviewed
• warn when duplicate, surface-overlap, or risk-summary usefulness cannot be measured from reviewed labels

Real corpus check

• corpus: 64 real instruction documents, 51 roots, 13 holdout documents, 3,359 lines
• current private manifest: 29 report findings, 0 reviewed, 29 unreviewed
• review item load remains 8.6 per 1,000 lines and risk finding load remains 19.4 per 1,000 lines
• strict mode now exits 1 until findings are manually labeled and usefulness targets are measurable

Verification

• npm test
• npm run validate:corpus
• node scripts/validate-corpus.mjs --manifest /tmp/rulemeter-real-corpus/manifest.json --format json --out /tmp/rulemeter-real-corpus/result.strict-label-coverage.no-text.json
• node scripts/validate-corpus.mjs --manifest /tmp/rulemeter-real-corpus/manifest.json --format json --strict --out /tmp/rulemeter-real-corpus/result.strict-label-coverage.strict.json exits 1 as expected
• npm run dogfood
• npm run pack:check
• npm run smoke:install
• npm audit --audit-level=high