Add immutable MITC4 drilling selection and direct director springs

include/fesa/elements/mitc4_drilling_mode.h

@@ -0,0 +1,26 @@
+#ifndef FESA_ELEMENTS_MITC4_DRILLING_MODE_H_
+#define FESA_ELEMENTS_MITC4_DRILLING_MODE_H_
+
+namespace fesa {
+
+/// @brief Identifies the immutable numerical MITC4 drilling formulation.
+enum class Mitc4DrillingMode { kDirectSpring, kLegacyCorotatedTwist };
+
+/// @brief Returns the stable storage name of a supported drilling formulation.
+constexpr const char* Mitc4DrillingModeName(Mitc4DrillingMode mode) noexcept {
+  switch (mode) {
+    case Mitc4DrillingMode::kDirectSpring:
+      return "direct-initial-director-spring";
+    case Mitc4DrillingMode::kLegacyCorotatedTwist:
+      return "legacy-corotated-twist";
+  }
+  return "";
+}
+
+namespace mitc4_drilling_internal {
+struct TestAccess;
+}  // namespace mitc4_drilling_internal
+
+}  // namespace fesa
+
+#endif  // FESA_ELEMENTS_MITC4_DRILLING_MODE_H_

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_;

include/fesa/elements/nonlinear_mitc4_shell.h

@@ -54,6 +54,8 @@ struct NonlinearMitc4InternalEquilibrium {
   Matrix drilling_metric{0U, 0U};
   double drilling_energy = 0.0;
   Vector force_directional_second{0U};
+  /// @brief Stores D2(f_drill)[v,v] separately; direct springs give exact zero.
+  Vector drilling_force_directional_second{0U};
   Vector drilling_constraint{0U};
   Matrix drilling_constraint_gradient{0U, 0U};
   std::array<Matrix, 4> drilling_constraint_hessian{
@@ -104,6 +106,11 @@ class NonlinearMitc4Shell {
       std::array<ShellNodeInitialFrame, 4> initial_frames,
       const ShellSection& section, const LinearElasticMaterial& material);
 
+  /// @brief Returns this immutable kernel's numerical drilling formulation.
+  [[nodiscard]] Mitc4DrillingMode DrillingMode() const noexcept {
+    return drilling_mode_;
+  }
+
   /// @brief Evaluates trial physical kinematics and fixed recovery rows.
   /// @param trial Global nodal translation and fixed-global rotation vectors.
   /// @return Physical recovery or a fail-closed trial diagnostic.
@@ -134,6 +141,7 @@ class NonlinearMitc4Shell {
 
  private:
   friend struct nonlinear_mitc4_internal::ConfigurationSnapshot;
+  friend struct mitc4_drilling_internal::TestAccess;
 
   /// @brief Retains validated fixed-reference data without analysis ownership.
   NonlinearMitc4Shell(std::array<Vector3, 4> coordinates,
@@ -155,6 +163,7 @@ class NonlinearMitc4Shell {
   double youngs_modulus_;
   double poisson_ratio_;
   double drilling_stiffness_;
+  Mitc4DrillingMode drilling_mode_;
   SourceLocation source_location_;
   std::string identity_;
 };