First attempt

include/kpn/port.hpp

@@ -0,0 +1,17 @@
+#pragma once
+#include <cstddef>
+
+namespace kpn {
+
+// Forward-declared so port handles can reference a node without including node.hpp
+template<typename NodeT, std::size_t Idx>
+struct InputPort {
+    NodeT& node;
+};
+
+template<typename NodeT, std::size_t Idx>
+struct OutputPort {
+    NodeT& node;
+};
+
+} // namespace kpn