feat(hex8): use reference-weighted mean-volume energy

include/fesa/elements/hex8_element.h

@@ -30,7 +30,7 @@ struct Hex8HighLowResponse {
   double strain_energy_low{0.0};
 };
 
-/// @brief Implements full-integration C3D8 linear and TL StVK mechanics.
+/// @brief Implements C3D8 full deviatoric and mean-volume Green energy.
 /// @note Reference geometry is owned and immutable; no trial is committed.
 class Hex8Element final : public Element {
  public:
@@ -53,14 +53,14 @@ class Hex8Element final : public Element {
 
   /// @brief Returns f=K0*u and infinitesimal tensor strain at eight points.
   Result<Hex8Response> EvaluateLinear(const Vector& displacement) const;
-  /// @brief Returns one certified StVK trial and its analytic energy Hessian.
+  /// @brief Returns one certified mean-Green TL trial and its 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.
+  /// @brief Evaluates mean-Green TL 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;
@@ -81,7 +81,7 @@ class Hex8Element final : public Element {
   };
   /// @brief Constructs an owned candidate before reference validation.
   Hex8Element(EntityIndex element_index, ElementDofLayout layout,
-              Coordinates coordinates, double lambda, double mu,
+              Coordinates coordinates, double kappa, double mu,
               std::optional<double> density, SourceLocation location);
   /// @brief Evaluates without mutating the cached reference or previous values.
   Result<Hex8Response> Evaluate(const Vector& displacement,
@@ -95,7 +95,7 @@ class Hex8Element final : public Element {
   EntityIndex element_index_;
   ElementDofLayout layout_;
   Coordinates coordinates_;
-  double lambda_;
+  double kappa_;
   double mu_;
   std::optional<double> density_;
   SourceLocation location_;