fix(signal): refuse filter designs whose coefficients overflow

Assisted-by: GLM 5.3 Flash

signal/filterdesign.go

@@ -286,9 +286,22 @@ func design(sh shape, proto prototype, w1, w2 float64) (b, a []float64, err erro
 	// reference follows from the roots and is no business of the
 	// scaling.
 	scale := proto.gain / cmplx.Abs(hd)
+	// An extreme order leaves the float64 range here as it does in the
+	// Butterworth pair: an infinite or vanished scale, or a coefficient
+	// past the range, is a refusal rather than a filter of zeros or NaN.
+	if math.IsNaN(scale) || math.IsInf(scale, 0) || scale == 0 {
+		return nil, nil, base.Errf("%s: the order overflows the coefficient arithmetic; use a lower order", name)
+	}
 	for i := range b {
 		b[i] *= scale
 	}
+	for _, poly := range [2][]float64{b, a} {
+		for _, v := range poly {
+			if math.IsNaN(v) || math.IsInf(v, 0) {
+				return nil, nil, base.Errf("%s: the order overflows the coefficient arithmetic; use a lower order", name)
+			}
+		}
+	}
 	return b, a, nil
 }