xo-callback xo-alloc: + GC copy callbacks + unique_ptr cbset support

xo-callback/include/xo/callback/callback_concept.hpp

@@ -0,0 +1,24 @@
+/* @file callback_concept.hpp
+ *
+ * author: Roland Conybeare, Aug 2025
+ */
+
+namespace xo {
+    namespace fn {
+        /** @concept callback_concept
+         *  @brief Concept for functors that can be used with @ref CallbackSetImpl
+         *
+         **/
+        template <typename Callback>
+        concept callback_concept = requires(Callback cb)
+        {
+            // no constraints on how callback is invoked.
+
+            { cb.notify_add_callback() };
+            { cb.notify_remove_callback() };
+        };
+
+    }
+}
+
+/* callback_concept.hpp */