Add challenge deadline fairness guard

[Davidrsdiaz]

/claim #18

Summary

• Adds a self-contained Scientific Bounty System deadline fairness guard for issue Scientific Bounty System #18.
• Validates sponsor deadline extensions, timezone-explicit cutoffs, equal notice to all eligible solver teams, late-submission acceptance, and freeze-window reopening before scoring or award release.
• Generates deterministic JSON, Markdown, SVG, and H.264 MP4 reviewer artifacts from synthetic data only.

Demo

• scientific-bounty-deadline-fairness-guard/reports/demo.mp4
• scientific-bounty-deadline-fairness-guard/reports/deadline-fairness-summary.svg

Validation

• node scientific-bounty-deadline-fairness-guard/test.js
• node scientific-bounty-deadline-fairness-guard/demo.js
• node scientific-bounty-deadline-fairness-guard/make-demo-video.js
• node --check scientific-bounty-deadline-fairness-guard/index.js
• node --check scientific-bounty-deadline-fairness-guard/sample-data.js
• node --check scientific-bounty-deadline-fairness-guard/demo.js
• node --check scientific-bounty-deadline-fairness-guard/test.js
• node --check scientific-bounty-deadline-fairness-guard/make-demo-video.js
• ffprobe -v error -select_streams v:0 -show_entries stream=codec_name,width,height,duration,nb_frames -show_entries format=size,duration -of default=noprint_wrappers=1 scientific-bounty-deadline-fairness-guard/reports/demo.mp4 -> H.264, 1280x720, 4.0s, 48 frames
• git diff --cached --check
• git diff --check

Safety

Synthetic fixtures only. No payment processor calls, private challenge data, credentials, external services, bank rails, or live SCIBASE systems are used.