xo-alloc2: register DArena facet + subsystem init

xo-alloc2/include/xo/alloc2/init_alloc2.hpp

@@ -0,0 +1,23 @@
+/** @file init_alloc2.hpp
+ *
+ *  @author Roland Conybeare, Feb 2026
+ **/
+
+#pragma once
+
+#include <xo/subsys/Subsystem.hpp>
+
+namespace xo {
+    /* tag to represent the xo-alloc2/ subsystem within ordered initialization */
+    enum S_alloc2_tag {};
+
+    template <>
+    struct InitSubsys<S_alloc2_tag> {
+        static void init();
+        static InitEvidence require();
+    };
+
+
+} /*namespace xo*/
+
+/* end init_alloc2.hpp */