formatValue now detects YAML-special tokens (true/false/null/yes/no/on/off/y/n/~)
and numeric-looking strings, quoting them via JSON.stringify so YAML parsers
cannot misread them as booleans or numbers. ISO date strings (YYYY-MM-DD) are
exempt and remain bare for readability. Adds four covering tests.
Co-Authored-By: Claude <noreply@anthropic.com>
