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

Assisted-by: GLM 5.3 Flash

linalg/decompositions_test.go

@@ -691,3 +691,18 @@ func TestCholeskyBlockedReconstruction(t *testing.T) {
 		}
 	}
 }
+
+// TestCholeskyRefusesNonFinite pins the refusal of a poisoned matrix:
+// the pivot test cannot see a NaN (it fails every comparison), so
+// without the gate the sweep would answer an all-NaN factor with a nil
+// error, where the sparse sibling refuses the same input.
+func TestCholeskyRefusesNonFinite(t *testing.T) {
+	nan := mustFromFloats(t, []float64{math.NaN(), 0, 0, 1}, 2, 2)
+	if _, err := Cholesky(nan); err == nil || !strings.Contains(err.Error(), "not finite") {
+		t.Fatalf("Cholesky(NaN): %v", err)
+	}
+	inf := mustFromFloats(t, []float64{0, 0, 0, math.Inf(1)}, 2, 2)
+	if _, err := Cholesky(inf); err == nil || !strings.Contains(err.Error(), "not finite") {
+		t.Fatalf("Cholesky(Inf): %v", err)
+	}
+}