Add reputation leaderboard eligibility guard

[vultuk]

/claim #15

Summary

• Adds reputation-leaderboard-eligibility-guard, a focused Community and User Reputation System slice for issue Community & User Reputation System #15.
• Validates whether domain, regional, or institutional reputation leaderboards are safe to publish before amplifying weak or gamed reputation signals.
• Checks evidence diversity, pending appeals, anonymous-review privacy, reciprocal endorsement concentration, institution dominance, stale reproducibility badges, and minimum accepted evidence.
• Emits reviewer artifacts: JSON audit packet, Markdown report, SVG summary, and a 12-second MP4 walkthrough.

Validation

• node reputation-leaderboard-eligibility-guard/test.js -> 5 reputation leaderboard eligibility guard tests passed
• node reputation-leaderboard-eligibility-guard/demo.js -> status=hold, eligible=3, held=1, blockers=4
• node --check reputation-leaderboard-eligibility-guard/index.js
• node --check reputation-leaderboard-eligibility-guard/sample-data.js
• node --check reputation-leaderboard-eligibility-guard/test.js
• node --check reputation-leaderboard-eligibility-guard/demo.js
• git diff --check

The demo status is intentionally hold: it shows the guard publishing the eligible ranks while holding one unsafe candidate with four explainable blockers.

BTC payout address for this bounty goal: bc1qa5acxqc3wldzxwjhe8a65gjp6n7dxmafm6qs4j