perf(hex8): cache immutable paired linear Hessian

include/fesa/elements/hex8_element.h

@@ -101,6 +101,11 @@ class Hex8Element final : public Element {
   SourceLocation location_;
   std::array<ReferencePoint, 8> points_{};
   Matrix stiffness_{24U, 24U};
+  // Optional immutable Hessian from the same paired quadrature graph. Failure
+  // to build it does not reject models supported by the original F64 paths.
+  bool paired_linear_tangent_ready_{false};
+  Matrix paired_linear_tangent_high_{24U, 24U};
+  Matrix paired_linear_tangent_low_{24U, 24U};
 };
 
 }  // namespace fesa