feat(analysis): execute solid generalized frequency pencil

include/fesa/elements/hex8_element.h

@@ -47,6 +47,9 @@ class Hex8Element final : public Element {
   Result<Hex8Response> EvaluateNonlinear(const Vector& displacement) const;
   /// @brief Returns the constant reference consistent mass; density required.
   Result<ElementStiffnessContribution> ComputeMass() const;
+  /// @brief Returns immutable positive reference Gauss volume weights.
+  /// @note Order is the kernel's fixed eight-point integration order.
+  std::array<double, 8> ReferenceWeights() const noexcept;
 
  private:
   /// @brief Caches shape, reference gradients and positive volume weights.