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

Assisted-by: GLM 5.3 Flash

linalg/decomp_test.go

@@ -5,8 +5,10 @@ package linalg
 
 import (
 	"math"
-	"sourcedock.dev/petrbalvin/tensor/internal/core"
+	"strings"
 	"testing"
+
+	"sourcedock.dev/petrbalvin/tensor/internal/core"
 )
 
 // TestSVDReconstruction checks that A = U · Σ · Vᵀ reconstructs A
@@ -477,3 +479,18 @@ func TestHouseholderVectorIntoLargeScale(t *testing.T) {
 		t.Fatalf("beta = %v for a zero vector, want 0", empty.beta)
 	}
 }
+
+// TestEigenRefusesNonFinite pins that a poisoned matrix never reads as
+// symmetric: the guard's comparison cannot see a NaN difference, so the
+// entry is refused outright, the way the sparse sibling's symmetry
+// check refuses it.
+func TestEigenRefusesNonFinite(t *testing.T) {
+	nan := mustFromFloats(t, []float64{math.NaN(), 0, 0, 1}, 2, 2)
+	if _, _, err := Eigen(nan); err == nil || !strings.Contains(err.Error(), "not finite") {
+		t.Fatalf("Eigen(NaN): %v", err)
+	}
+	inf := mustFromFloats(t, []float64{math.Inf(1), 0, 0, 1}, 2, 2)
+	if _, _, err := Eigen(inf); err == nil || !strings.Contains(err.Error(), "not finite") {
+		t.Fatalf("Eigen(Inf): %v", err)
+	}
+}