- Document backward compat rationale in cyrillic.json - Extract magic number 4 to MaxExpansionRatio constant - Add fail-fast assertion for missing golden mappings file - Add edge case tests (control chars, whitespace, emoji, empty mappings) - Handle original converter buffer overflow bugs in golden tests 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>