Add collaborative notification visibility guard

[ethanmillerinvestments-code]

/claim #12

Summary

• Adds collab-notification-visibility-guard/, a focused issue Real-time collaborative research editor & interface #12 slice for real-time collaborative research editor notification fanout.
• Evaluates in-app, email, push, and webhook notification routes against recipient role, scopes, channel safety, digest receipts, and privacy permissions.
• Blocks or sanitizes blinded reviewer identities, private section titles, embargoed anchors, restricted notebook paths, and private collaborator notes before they leave the editor.
• Emits deterministic JSON, Markdown, SVG, and H.264 MP4 demo artifacts using synthetic local data only.

Non-overlap

This is not another broad editor foundation, offline conflict resolver, notebook workbench, reference formatter, authorship governance module, freeze/recovery lane, discussion sidebar audit, autosave recovery module, round-trip fidelity checker, review decision ledger, task dependency guard, equation/figure anchor guard, notebook kernel lease guard, presence privacy guard, accessibility parity guard, manuscript evidence-binding guard, or embargo release guard. It focuses specifically on notification and digest payload visibility after collaborative editor events.

Validation

• npm run check
• npm test
• npm run demo
• ffprobe -v error -show_entries stream=codec_name,width,height,pix_fmt -show_entries format=duration,size -of default=noprint_wrappers=1 reports/demo.mp4
• git diff --cached --check

MP4 metadata: H.264, 1280x720, yuv420p, 6s, 10808 bytes.