Add immutable MITC4 drilling selection and direct director springs

src/fesa/elements/nonlinear_mitc4_configuration_internal.h

@@ -12,6 +12,7 @@ namespace fesa::nonlinear_mitc4_internal {
 /// key.
 struct ConfigurationSnapshot {
   std::array<double, 64> values{};
+  Mitc4DrillingMode drilling_mode = Mitc4DrillingMode::kDirectSpring;
   SourceLocation source{};
   std::string identity;