Merge reviewed MKL shell products and complete-trial reuse

include/fesa/elements/nonlinear_mitc4_shell.h

@@ -15,6 +15,10 @@
 
 namespace fesa {
 
+namespace nonlinear_mitc4_internal {
+struct ConfigurationSnapshot;
+}  // namespace nonlinear_mitc4_internal
+
 /// @brief Stores one four-node nonlinear shell trial in global coordinates.
 struct NonlinearMitc4TrialDofs {
   std::array<Vector3, 4> displacement;
@@ -129,6 +133,8 @@ class NonlinearMitc4Shell {
                                      const Vector3& force_acceleration) const;
 
  private:
+  friend struct nonlinear_mitc4_internal::ConfigurationSnapshot;
+
   /// @brief Retains validated fixed-reference data without analysis ownership.
   NonlinearMitc4Shell(std::array<Vector3, 4> coordinates,
                       std::array<Matrix, 4> initial_frames,