fix(stats): hold the Student-t tails past t-squared overflow

Assisted-by: GLM 5.3 Flash

stats/noncentral.go

@@ -304,7 +304,14 @@ func NoncentralTCDF(t float64, df int, delta float64) (float64, error) {
 		magnitude = -t
 		shift = -delta
 	}
-	x := magnitude * magnitude / (magnitude*magnitude + float64(df))
+	magnitude2 := magnitude * magnitude
+	x := magnitude2 / (magnitude2 + float64(df))
+	if math.IsInf(magnitude2, 1) {
+		// A finite t whose square overflows drove the quotient through
+		// Inf/Inf into a NaN the incomplete beta refused under its own
+		// name. The beta argument's limit there is exactly 1.
+		x = 1
+	}
 	if x == 0 {
 		// t = 0: the value collapses to Φ(−δ) exactly.
 		return NormalCDF(-delta), nil