test(hex8): pin original overflow diagnostic priorities

tests/unit/elements/hex8_element_test.cpp

@@ -811,6 +811,36 @@ TEST(Hex8Element, FiniteInputsThatOverflowDerivedArithmeticFailClosed) {
             std::string::npos);
 }
 
+TEST(Hex8Element, BackendOperandRejectionRetainsOriginalStiffnessDiagnostic) {
+  Coordinates coordinates{};
+  for (std::size_t a = 0; a < 8; ++a)
+    for (std::size_t i = 0; i < 3; ++i)
+      coordinates[a][i] = 0.01 * kSigns[a][i];
+  // Original native probe: finite moduli/volume, but mu*gradient overflows.
+  // The failed diagnostic candidate must not become a scalar success fallback.
+  const auto element = Make(coordinates, 1e308, -0.5);
+  ASSERT_FALSE(element.HasValue());
+  EXPECT_EQ(element.GetStatus().Diagnostics()[0].code, "invalid-hex8");
+  EXPECT_EQ(element.GetStatus().Diagnostics()[0].message,
+            "Hex8 reference stiffness is nonfinite.");
+}
+
+TEST(Hex8Element, TensorFailureRetainsPriorityOverEnergyAndTangentOverflow) {
+  Coordinates coordinates{};
+  for (std::size_t a = 0; a < 8; ++a)
+    for (std::size_t i = 0; i < 3; ++i)
+      coordinates[a][i] = 0.01 * kSigns[a][i];
+  const auto element = Make(coordinates, 1e200, -0.5);
+  ASSERT_TRUE(element.HasValue());
+  const auto displacement = Affine(coordinates,
+      Tensor{{{1e100, 0, 0}, {0, 1e100, 0}, {0, 0, 1e100}}});
+  const auto response = element.Value().EvaluateNonlinear(displacement);
+  ASSERT_FALSE(response.HasValue());
+  EXPECT_EQ(response.GetStatus().Diagnostics()[0].code, "invalid-hex8");
+  EXPECT_EQ(response.GetStatus().Diagnostics()[0].message,
+            "Hex8 trial tensor is nonfinite.");
+}
+
 // Independent Gauss-Legendre abscissae from polynomial root iteration. This
 // test does not call the kernel's shapes, Jacobian, inverse or quadrature.
 std::vector<std::array<double, 2>> LegendreRule(unsigned count) {