Refuse a rows weight too close to 0 to tell from it, by the rule a JSON option uses

internal/grammar/calc.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"slices"
 	"strconv"
+	"strings"
 	"unicode"
 )
 
@@ -177,3 +178,10 @@ func (p *calcParser) ident() (CalcNode, error) {
 	}
 	return CalcVar{name, at}, nil
 }
+
+// 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")
+}