fix(linalg): refuse non-finite input in dense Cholesky, Eigen and rank-one updates

Assisted-by: GLM 5.3 Flash

linalg/decomp2.go

@@ -706,6 +706,18 @@ func Eigen(a *core.Array) (values, vectors *core.Array, err error) {
 	}
 	n := a.Shape()[0]
 	mat := denseFloats(a, n, n)
+	// A non-finite entry slips through the symmetry test below: the
+	// difference of two NaNs never exceeds the tolerance, so a poisoned
+	// matrix reads as symmetric and the sweep hands back a NaN spectrum
+	// with a nil error. The sparse twin refuses the same input, and so
+	// does this one.
+	for i := range n {
+		for j := range n {
+			if math.IsNaN(mat[i*n+j]) || math.IsInf(mat[i*n+j], 0) {
+				return nil, nil, base.Errf("Eigen: entry [%d,%d] is not finite", i, j)
+			}
+		}
+	}
 	// A matrix outside the safe window would drive the reflector norms
 	// and the sweep's squared accumulation past the representable range;
 	// the tridiagonalisation and the QR sweep run on it scaled into the