Add artifact retention tombstone ledger

[tuanadr]

/claim #14

Summary

• Adds a self-contained hosted artifact retention and tombstone ledger for scientific data/code hosting.
• Blocks disposal for active policy holds, unelapsed retention windows, missing checksum manifests, DOI/citation tombstone gaps, and destructive delete requests for cited artifacts.
• Emits deterministic tombstone ledger packets with decisions, metadata requirements, audit digest, JSON/Markdown/SVG output, and a short MP4 demo.

Demo artifacts

• artifact-retention-tombstone-ledger/demo-output/tombstone-ledger-packet.json
• artifact-retention-tombstone-ledger/demo-output/disposal-review.md
• artifact-retention-tombstone-ledger/demo-output/demo.svg
• artifact-retention-tombstone-ledger/demo-output/demo.mp4

Validation

• node artifact-retention-tombstone-ledger/test.js
• node artifact-retention-tombstone-ledger/demo.js
• node --check artifact-retention-tombstone-ledger/index.js
• node --check artifact-retention-tombstone-ledger/test.js
• node --check artifact-retention-tombstone-ledger/demo.js
• node --check artifact-retention-tombstone-ledger/sample-data.js
• ffprobe -v error -show_entries format=duration,size -show_entries stream=codec_name,width,height -of default=noprint_wrappers=1 artifact-retention-tombstone-ledger/demo-output/demo.mp4
• git diff --check
• git diff --cached --check