Add challenge clarification freeze guard

[HunterCML]

/claim #18

Summary

• add dependency-free challenge clarification freeze guard for the scientific bounty system
• check material sponsor Q&A, post-cutoff rule changes, amendment links, and all-solver broadcasts
• emit freeze packet, notification digests, reviewer artifacts, SVG preview, and H.264 demo video

Validation

• node challenge-clarification-freeze-guard/test.js
• node challenge-clarification-freeze-guard/demo.js
• node --check challenge-clarification-freeze-guard/index.js
• node --check challenge-clarification-freeze-guard/test.js
• node --check challenge-clarification-freeze-guard/demo.js
• ffprobe -v error -show_entries format=duration,size -show_entries stream=codec_name,width,height -of default=noprint_wrappers=1 challenge-clarification-freeze-guard/demo.mp4
• git diff --check