Card content kind: 'text' (default when null), 'code', 'formula', or 'diagram'. Drives code-aware rendering + exercise input (monospace editor for code/formula). EXP-039: a closed Literal so the generated JSON-Schema / TS types carry the exact union (was a free str gated by a runtime validator).
Card content kind: 'text' (default when null), 'code', 'formula', or 'diagram'. Drives code-aware rendering + exercise input (monospace editor for code/formula). EXP-039: a closed
Literalso the generated JSON-Schema / TS types carry the exact union (was a freestrgated by a runtime validator).