feat(linear-static-mitc4-shell): step 4 - mitc4-stiffness-drilling

include/fesa/elements/mitc4_shell.hpp

@@ -5,6 +5,7 @@
 #include "fesa/model/model_types.hpp"
 
 #include <array>
+#include <string>
 
 namespace fesa {
 
@@ -30,8 +31,16 @@ struct Mitc4QuadraturePoint {
     double weight;
 };
 
-// Concrete small-rotation MITC4 kinematics and material value kernel. Global
-// equation ownership, drilling stiffness, recovery, and assembly remain outside.
+struct Mitc4Stiffness {
+    Matrix physicalLocal20;
+    Matrix physicalGlobal24;
+    Matrix drillingGlobal24;
+    Matrix stabilizedGlobal24;
+    double drillingStiffness;
+};
+
+// Concrete small-rotation MITC4 kinematics, constitutive, and stiffness kernel.
+// Global equation ownership, physical recovery, and assembly remain outside.
 class Mitc4Shell {
 public:
     static Result<Mitc4Shell> create(
@@ -63,6 +72,7 @@ public:
     [[nodiscard]] Matrix membraneSectionMatrix() const;
     [[nodiscard]] Matrix bendingSectionMatrix() const;
     [[nodiscard]] Matrix transverseShearSectionMatrix() const;
+    [[nodiscard]] Result<Mitc4Stiffness> stiffness() const;
 
 private:
     using Vector3 = std::array<double, 3>;
@@ -82,7 +92,9 @@ private:
         Vector3 normalCandidate,
         double thickness,
         double youngsModulus,
-        double poissonRatio);
+        double poissonRatio,
+        SourceLocation sourceLocation,
+        std::string identity);
 
     bool evaluateGeometry(
         double xi,
@@ -107,6 +119,8 @@ private:
     double thickness_;
     double youngsModulus_;
     double poissonRatio_;
+    SourceLocation sourceLocation_;
+    std::string identity_;
 };
 
 } // namespace fesa