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

Assisted-by: GLM 5.3 Flash

linalg/decomp.go

@@ -48,7 +48,19 @@ func Cholesky(a *core.Array) (*core.Array, error) {
 		return nil, base.Errf("Cholesky: needs a square 2-D matrix, got shape %s", base.ShapeText(a.Shape()))
 	}
 	n := a.Shape()[0]
-	lMat, err := denseCholFactor(denseFloats(a, n, n), n)
+	aMat := denseFloats(a, n, n)
+	// A non-finite entry is neither positive nor definite, but the pivot
+	// test below cannot see it: NaN fails every comparison, so the sweep
+	// would return an all-NaN factor 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(aMat[i*n+j]) || math.IsInf(aMat[i*n+j], 0) {
+				return nil, base.Errf("Cholesky: entry [%d,%d] is not finite", i, j)
+			}
+		}
+	}
+	lMat, err := denseCholFactor(aMat, n)
 	if err != nil {
 		return nil, err
 	}