Add collaborative local cache privacy guard

[shaiananvari8]

/claim #12

Summary

• Adds collab-local-cache-privacy-guard, a dependency-free synthetic reviewer module for local/browser autosave cache privacy in the collaborative editor.
• Checks restricted manuscript/data excerpts, anonymous-review identity leakage, embargoed titles, stale notebook outputs, private collaborator notes, encryption, storage backend, shared-device retention, and weak device-lock posture before cache persistence or restore.
• Generates reviewer artifacts under collab-local-cache-privacy-guard/reports/, including demo.avi.

Validation

• npm run check
• npm test
• npm run demo
• npm run demo:video
• git diff --check
• git diff --cached --check

Demo

The short demo video is committed at collab-local-cache-privacy-guard/reports/demo.avi.

Scope Boundary

This is intentionally separate from autosave recovery, operation replay, offline conflict rebasing, presence privacy, notification visibility, reference merging, section lock recovery, and broad editor foundation work. It focuses only on local cache privacy and retention safety before collaborative autosave payloads are persisted or offered for restore.