feat(hex8): preserve full high-low state in dynamic element responses

include/fesa/elements/hex8_element.h

@@ -20,6 +20,16 @@ struct Hex8Response {
   std::array<SolidIntegrationPointRow, 8> points{};
 };
 
+/// @brief Normalized full-state response for Solid HHT high+low arithmetic.
+/// @note Point rows are rounded observables; forces, Hessian and energy retain
+/// their low components until global assembly and physical validation.
+struct Hex8HighLowResponse {
+  Hex8Response high;
+  Vector internal_force_low{24U};
+  Matrix tangent_low{24U, 24U};
+  double strain_energy_low{0.0};
+};
+
 /// @brief Implements full-integration C3D8 linear and TL StVK mechanics.
 /// @note Reference geometry is owned and immutable; no trial is committed.
 class Hex8Element final : public Element {
@@ -45,6 +55,15 @@ class Hex8Element final : public Element {
   Result<Hex8Response> EvaluateLinear(const Vector& displacement) const;
   /// @brief Returns one certified StVK trial and its analytic energy Hessian.
   Result<Hex8Response> EvaluateNonlinear(const Vector& displacement) const;
+  /// @brief Evaluates linear energy/force/Hessian on the complete paired state.
+  /// @note Uses the same reference-relative gradient operator as the paired TL
+  /// path. Inputs must have 24 finite normalized high+low translations.
+  Result<Hex8HighLowResponse> EvaluateLinearHighLow(
+      const Vector& high, const Vector& low) const;
+  /// @brief Evaluates certified TL StVK mechanics without collapsing low state.
+  /// @note Failure preserves immutable reference data and all caller state.
+  Result<Hex8HighLowResponse> EvaluateNonlinearHighLow(
+      const Vector& high, const Vector& low) const;
   /// @brief Returns the constant reference consistent mass; density required.
   Result<ElementStiffnessContribution> ComputeMass() const;
   /// @brief Returns immutable positive reference Gauss volume weights.
@@ -67,6 +86,9 @@ class Hex8Element final : public Element {
   /// @brief Evaluates without mutating the cached reference or previous values.
   Result<Hex8Response> Evaluate(const Vector& displacement,
                                 bool nonlinear) const;
+  /// @brief Uses one paired gradient for energy, force and analytic Hessian.
+  Result<Hex8HighLowResponse> EvaluateHighLow(
+      const Vector& high, const Vector& low, bool nonlinear) const;
   /// @brief Attaches stable source context to a model failure.
   Status Failure(const char* message) const;