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")
+}

internal/rows/rows.go

@@ -218,9 +218,13 @@ func (t *Table[O]) sumWeights() error {
 	t.weights, t.cum = make([]float64, t.Len()), make([]float64, t.Len())
 	total := 0.0
 	for r := range t.cum {
-		w, err := strconv.ParseFloat(t.Cell(r, t.weightIndex), 64)
+		cell := t.Cell(r, t.weightIndex)
+		w, err := strconv.ParseFloat(cell, 64)
 		if err != nil || math.IsInf(w, 0) || math.IsNaN(w) || w < 0 {
-			return fmt.Errorf("%s line %d: weight %q is not a number of 0 or more", t.file, r+2, t.Cell(r, t.weightIndex))
+			return fmt.Errorf("%s line %d: weight %q is not a number of 0 or more", t.file, r+2, cell)
+		}
+		if grammar.Underflows(cell, w) {
+			return fmt.Errorf("%s line %d: weight %s is too close to 0 to tell from it", t.file, r+2, cell)
 		}
 		total += w
 		t.weights[r], t.cum[r] = w, total