Add collaborative alt text export guard

[AlonePenguin]

/claim #12

Summary

• Added a dependency-free collaborative-alt-text-export-guard module for the real-time collaborative research editor bounty.
• Evaluates publication-ready figures and tables for current alt text, long descriptions, table header scopes, color-only encodings, stale source hashes, locked-section approvals, unresolved accessibility comments, and private reviewer/path/data leaks.
• Includes clean/revision/hold synthetic packets plus JSON, Markdown, SVG, and MP4 reviewer artifacts under collaborative-alt-text-export-guard/reports/.

Non-overlap

This is focused on manuscript content accessibility at export time. It is distinct from prior broad editor, control-accessibility, figure/table review, citation/reference, equation-anchor, mode-toggle, clipboard/import, data-availability, and table-formula guard slices.

Verification

• npm run check
• npm test
• npm run demo
• npm run verify-video -> H.264, 960x540, 3.000000 seconds
• git diff --check

Attempt marker: #12 (comment)