Add immutable MITC4 drilling selection and direct director springs

include/fesa/elements/mitc4_shell.h

@@ -8,6 +8,7 @@
 #include "fesa/core/status.h"
 #include "fesa/elements/element.h"
 #include "fesa/elements/mitc4_shell_definition.h"
+#include "fesa/elements/mitc4_drilling_mode.h"
 #include "fesa/materials/isotropic_linear_elastic_material.h"
 #include "fesa/math/matrix.h"
 #include "fesa/math/vector.h"
@@ -92,6 +93,11 @@ class Mitc4Shell final : public Element {
       std::array<std::array<double, 3>, 4> initial_directors,
       const ShellSection& section, const LinearElasticMaterial& material);
 
+  /// @brief Returns this immutable kernel's production drilling formulation.
+  [[nodiscard]] Mitc4DrillingMode DrillingMode() const noexcept {
+    return drilling_mode_;
+  }
+
   /// @brief Returns the factory-bound MITC4 DOF order.
   const ElementDofLayout& DofLayout() const noexcept override;
 
@@ -163,6 +169,7 @@ class Mitc4Shell final : public Element {
 
  private:
   friend class ElementFactory;
+  friend struct mitc4_drilling_internal::TestAccess;
 
   /// @brief Binds Domain identity after the numerical candidate is valid.
   void BindRuntime(ElementDofLayout layout, EntityIndex element_index,
@@ -203,6 +210,7 @@ class Mitc4Shell final : public Element {
   double thickness_;
   double youngs_modulus_;
   double poisson_ratio_;
+  Mitc4DrillingMode drilling_mode_;
   SourceLocation source_location_;
   std::string identity_;
   ElementDofLayout dof_layout_;