Add comprehensive documentation about the code factoring mistake made
during addition worksheet preview implementation. Document the correct
pattern: extract to shared utility instead of copy/paste.
- Explain why "factor not fork" is critical
- Show correct pattern: extract to shared function
- Show wrong pattern: copy/paste between files
- Add red flags to watch for
- Document specific example from this implementation
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
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