Add collaborative presence privacy guard

[vultuk]

/claim #12

Summary

• Adds collab-presence-privacy-guard, a focused Real-Time Collaborative Editor slice for issue Real-time collaborative research editor & interface #12.
• Validates live cursor and user-presence broadcasts before they are sent to collaborators.
• Prevents private section titles, embargoed cell anchors, anonymous reviewer identities, locked dataset areas, and restricted collaborator locations from leaking through real-time UI metadata.
• Emits sanitized room snapshots, blocked cursor broadcasts, reviewer actions, JSON audit packet, Markdown report, SVG summary, and a 12-second MP4 walkthrough.

Validation

• node collab-presence-privacy-guard/test.js -> 5 collaborative presence privacy guard tests passed
• node collab-presence-privacy-guard/demo.js -> status=hold, safe=0, warnings=0, blocked=5
• node --check collab-presence-privacy-guard/index.js
• node --check collab-presence-privacy-guard/sample-data.js
• node --check collab-presence-privacy-guard/test.js
• node --check collab-presence-privacy-guard/demo.js
• git diff --check

The demo status is intentionally hold: it demonstrates the guard blocking unsafe live presence broadcasts where cursor anchors would expose embargoed data, private notebook cells, or anonymous reviewer identity.

BTC payout address for this bounty goal: bc1qa5acxqc3wldzxwjhe8a65gjp6n7dxmafm6qs4j