Add artifact preview cache guard

[swhan0329]

Summary

• Adds a self-contained artifact preview cache and version guard for issue Scientific/Engineering Data & Code Hosting #14.
• Classifies hosted datasets, code, images, media, models, and supplemental artifacts into preview lanes.
• Detects stale previews when content hashes, upload versions, schema versions, or metadata digests drift.
• Holds unsafe previews that expose sensitive fields and emits dataset diff summaries, FAIR warnings, preview plans, and a deterministic audit digest.
• Includes a short H.264 demo artifact at artifact-preview-cache-guard/demo.mp4.

Validation

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

/claim #14

Built with AI assistance; reviewed and tested before submission.

[swhan0329]

@algora-pbc /claim #14

[swhan0329]

Fresh local verification for the #14 claim.

Commands rerun on codex/artifact-preview-cache-guard:

node artifact-preview-cache-guard/test.js
node artifact-preview-cache-guard/demo.js
node --check artifact-preview-cache-guard/index.js
node --check artifact-preview-cache-guard/test.js
node --check artifact-preview-cache-guard/demo.js
ffprobe -v error -show_entries format=duration,size -show_entries stream=codec_name,width,height -of default=noprint_wrappers=1 artifact-preview-cache-guard/demo.mp4
git diff --check
git diff --cached --check

Result: all passed. Demo output still shows status: blocked, artifacts checked: 2, refresh count: 1, blocked count: 1, blocked reason participant-roster preview exposes sensitive fields: email, digest d7046f2b9b455e54.... ffprobe confirms the demo video is H.264, 1280x720, 5.0s, 43635 bytes.