Add resumable upload checkpoint guard

[haki203]

Summary

• Adds a distinct resumable upload checkpoint guard slice for issue Scientific/Engineering Data & Code Hosting #14.
• Validates multipart dataset/notebook/supplement upload checkpoints before durable artifact commit.
• Flags missing chunks, checksum mismatches, size drift, missing final manifest hashes, missing metadata schemas, and expired resume state.
• Emits deterministic reviewer decisions, required actions, JSON/Markdown/SVG reports, and a short MP4 demo artifact from synthetic data only.

Validation

• node resumable-upload-checkpoint-guard/test.js
• node resumable-upload-checkpoint-guard/demo.js
• node --check resumable-upload-checkpoint-guard/index.js
• node --check resumable-upload-checkpoint-guard/test.js
• node --check resumable-upload-checkpoint-guard/demo.js
• node --check resumable-upload-checkpoint-guard/sample-data.js
• ffmpeg decode check for resumable-upload-checkpoint-guard/reports/demo.mp4
• git diff --cached --check

No secrets, private datasets, storage provider credentials, external APIs, or network calls are used by the module.

/claim #14