Add collaborative reference merge guard

[haki203]

/claim #12

Summary:

• Adds a distinct collaborative-reference-merge-guard/ slice for issue Real-time collaborative research editor & interface #12.
• Validates concurrent Zotero, BibTeX, and EndNote-style reference library merges before collaborative manuscript export.
• Detects DOI/PMID duplicate records, citation-key collisions, stale in-text citation anchors, locked-reference edits, incomplete source metadata, and unresolved merge conflicts.
• Generates deterministic JSON, Markdown, SVG, and MP4 reviewer artifacts from synthetic data only.

Validation:

• node --test collaborative-reference-merge-guard/test.js -> 3 tests passed.
• node --check collaborative-reference-merge-guard/index.js, test.js, sample-data.js, demo.js -> exit 0.
• FFMPEG_PATH=C:\Users\tho38\AppData\Local\Temp\codex-ffmpeg-static\node_modules\ffmpeg-static\ffmpeg.exe node collaborative-reference-merge-guard/demo.js -> scenarios=3, hold-export=1, merge-review=1, export-ready=1.
• ffmpeg -v error -i collaborative-reference-merge-guard/reports/demo.mp4 -f null NUL -> exit 0.
• git diff --check -> exit 0.

Safety:
Synthetic fixtures only. No private manuscripts, credentials, browser cookies, payment data, Zotero/EndNote accounts, Crossref/PubMed calls, DOI registry calls, or external services.