feat: add solver core skeleton

src/fesa/fem/dof_key.hpp

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <fesa/core/ids.hpp>
+#include <fesa/model/boundary_condition.hpp>
+
+namespace fesa::fem {
+
+struct DofKey {
+    core::NodeId node_id;
+    model::DofComponent component;
+};
+
+inline bool operator==(const DofKey& lhs, const DofKey& rhs)
+{
+    return lhs.node_id.value == rhs.node_id.value && lhs.component == rhs.component;
+}
+
+} // namespace fesa::fem