Drop the CLI's no-data hint, keep the number underflow rule in grammar/number.go, and name the JSON a template may be

internal/grammar/number.go

@@ -0,0 +1,10 @@
+package grammar
+
+import "strings"
+
+// Underflows reports a number written as text that reads as f = 0 though a digit before its
+// exponent is not 0, as 1e-400 does: no float64 holds the number written.
+func Underflows(text string, f float64) bool {
+	mantissa, _, _ := strings.Cut(strings.ToLower(text), "e")
+	return f == 0 && strings.ContainsAny(mantissa, "123456789")
+}