feat(elements): implement certified full-integration Hex8 kernel

include/fesa/elements/element.h

@@ -11,6 +11,7 @@
 #include "fesa/math/matrix.h"
 #include "fesa/math/vector.h"
 #include "fesa/results/result_records.h"
+#include "fesa/results/solid_result_records.h"
 
 namespace fesa {
 
@@ -52,7 +53,8 @@ struct ShellElementResultRows {
 
 /// @brief Selects the physical recovery shape of one runtime element.
 using ElementResultPayload =
-    std::variant<BeamElementResultRows, ShellElementResultRows>;
+    std::variant<BeamElementResultRows, ShellElementResultRows,
+                 SolidElementResultRows>;
 
 /// @brief Carries stable source identity with one typed recovery payload.
 struct ElementResultBundle {