Add comment anchor drift gate

[wbo68410-lgtm]

/claim #12

Summary

• Adds a distinct comment anchor drift gate for the real-time collaborative research editor.
• Reviews inline comments and annotations after document edits for missing quote anchors, section drift, stale notebook output anchors, large movement requiring confirmation, and invalid re-anchor quotes.
• Includes synthetic collaborative edit packets plus deterministic JSON, Markdown, and SVG reviewer artifacts.

Non-overlap

This focuses specifically on preserving inline comment and annotation correctness after collaborative edits. It is separate from broad collaborative editor modules, autosave recovery, local cache privacy, clipboard import, suggestion acceptance provenance, accessibility parity, reference merge, and notebook reproducibility guards.

Verification

• node real-time-collaborative-editor/comment-anchor-drift-gate/test.js
• node real-time-collaborative-editor/comment-anchor-drift-gate/demo.js
• git diff --check

All data is synthetic; no private documents, collaborator identities, live editor sessions, kernels, or external services are used.

Closes #12