Record 40-paper deployment handoff

[estelledc]

Summary

• record PR Add 40 arXiv paper study cards #31 and GitHub Pages deployment evidence for the 40-paper study batch
• mark the 40-paper and prior 4-paper epochs complete in SESSION-HANDOFF.md
• clarify that the next writer requires a new explicit backlog or external delta

Verification

• STUDY_CHANGED_FROM=9eadc605426eed61b7c4ffcc9377d0230b143381 npm run verify:ci
• npm run audit:doc-lifecycle
• git diff --check