feat(ecs): implement basic ECS

include/libcatboy/ecs/fwd.hpp

@@ -0,0 +1,20 @@
+#ifndef LIBCATBOY_ECS_FWD_HPP
+#define LIBCATBOY_ECS_FWD_HPP
+
+#include <concepts>
+#include <cstdint>
+
+namespace libcatboy {
+namespace ecs {
+
+using entity = std::uint32_t;
+
+template <std::integral Entity = entity>
+class basic_registry;
+
+using registry = basic_registry<>;
+
+} // namespace ecs
+} // namespace libcatboy
+
+#endif // LIBCATBOY_ECS_FWD_HPP