Add challenge benchmark holdout leakage guard

[zergzorg]

@algora-pbc /claim #18

Summary

• Adds a self-contained challenge benchmark holdout leakage guard for issue Scientific Bounty System #18.
• Validates private holdout scoring readiness by checking holdout hashes, release channels, deadline ordering, solver workspace grants, public baseline hash overlap, sponsor exceptions, and submission artifact hashes.
• Emits score/review/quarantine decisions, scoring actions, deterministic audit digests, and JSON/Markdown/SVG/MP4 review artifacts.

Scope boundaries

• Uses synthetic challenge metadata only.
• Does not handle payout, solver identity, KYC, payment routing, private files, or external accounts.
• Avoids overlap with existing rubric readiness, milestone progress, evidence freeze, IP redaction, payout eligibility, reviewer consensus, solver prequalification, data-room access, clarification freeze, award transparency, deliverable acceptance, and reproducibility environment slices.

Validation

• npm run check
• npm test
• npm run demo
• ffprobe -v error -show_entries format=duration,size -show_entries stream=codec_name,width,height -of default=noprint_wrappers=1 reports/demo.mp4
• git diff --check

[zergzorg]

Claim registration follow-up:

@algora-pbc /claim #18