Per ES, ToPrimitive only accepts strings/numbers/booleans/null
/undefined as primitives — objects AND functions trigger the next
step. Was treating function returns from toString/valueOf as
primitives (recursing to extract a string), so toString returning
a function didn't fall through to valueOf. Widened the dict-only
check to (or (= type "dict") (js-function? result)) in both
js-to-string and js-to-number ToPrimitive paths.
built-ins/String: 85/99 → 86/99. conformance.sh: 148/148.