ShortformWriter routes every short-form string through AttrWriter::cleanValue()
with the curated field kind (new optional kind override), exposes the color
check for background_color and guards builder-only paths on removal too.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7
