Function scalar_test_operand_error 

pub fn scalar_test_operand_error(
    op_symbol: &str,
    value: &Value,
) -> Option<String>

Decision E: scalar-only test operators (-z/-n, =~/!~, ordering </>/<=/>=, and numeric -eq/-ne/-gt/-lt/-ge/-le) refuse a collection operand loudly rather than falling through a stringify/silent path. ==/!= already error via values_equal; in/not in are the one operator family that legitimately takes a collection operand and must never call this. None for scalars. Shared by the sync eval_test (interpreter/eval.rs) and the async eval_test_async (kernel.rs) so the two [[ ]] evaluators can’t drift apart on this guard.