fix: preserve concurrent case additions

[rogerchappel]

Summary

• serialize additions to one case with a case-local lock and bounded retry window
• replace index.json atomically and remove newly written entries if index persistence fails
• add a 12-process CLI regression covering show, search, export, and orphan detection
• document locking, retry timeout, and atomic reader behavior

Commits

• af9ad3b test: cover concurrent CLI additions
• f0960ab fix: serialize case index updates
• 1ed7809 docs: explain concurrent writer behavior

Verification

• npm run release:check
• bash scripts/validate.sh
• git diff --check origin/main..HEAD

All checks passed locally on Node.js 22.