Add review load validation metric

[heznpc]

Summary

• add review item load and underlying risk finding load metrics to corpus validation
• warn when report findings exceed the review-item budget per 1,000 instruction lines
• document the review-load target alongside surface-overlap and risk-summary usefulness targets

Real corpus check

• corpus: 64 real instruction documents, 51 roots, 13 holdout documents, 3,359 lines
• default run: 29 report findings, review item load 8.6 per 1,000 lines, risk finding load 19.4 per 1,000 lines
• with --experimental-similar: 52 report findings, review item load 15.5 per 1,000 lines, risk finding load 19.4 per 1,000 lines

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.review-load.no-text.json
• node scripts/validate-corpus.mjs --manifest /tmp/rulemeter-real-corpus/manifest.json --format json --out /tmp/rulemeter-real-corpus/result.review-load-similar.no-text.json --experimental-similar
• npm run dogfood
• npm run pack:check
• npm run smoke:install
• npm audit --audit-level=high