xo-gc: refactor: demote GCObjectVisitor to GCObjectStore

No longer needed by DX1Collector
Also retires utest/MockCollector

include/xo/gc/DGCObjectStoreVisitor.hpp

@@ -0,0 +1,49 @@
+/** @file DGCObjectStoreVisitor.hpp
+ *
+ *  @author Roland Conybeare, Apr 2026
+ **/
+
+#pragma once
+
+#include <xo/alloc2/GCObjectVisitor.hpp>
+#include <xo/alloc2/VisitReason.hpp>
+#include <xo/alloc2/Generation.hpp>
+#include <xo/alloc2/role.hpp>
+#include <xo/arena/AllocInfo.hpp>
+#include <xo/facet/obj.hpp>
+
+namespace xo {
+    namespace mm {
+
+        class GCObjectStore; // see GCObjectStore.hpp
+        class AGCObject; // see AGCObject.hpp
+
+        /** @brief visitor shim for GCObjectStore
+         *
+         *  For a GC cycle, remembers which generations
+         *  are being collected
+         **/
+        class DGCObjectStoreVisitor {
+        public:
+            DGCObjectStoreVisitor(GCObjectStore * gcos, Generation upto);
+            
+            template <typename AFacet = AGCObjectVisitor>
+            obj<AFacet,DGCObjectStoreVisitor> ref() { return obj<AFacet,DGCObjectStoreVisitor>(this); }
+
+            Generation generation_of(Role r, const void * addr) const noexcept;
+            AllocInfo alloc_info(void * mem) const noexcept;
+
+            void visit_child(VisitReason reason, AGCObject * lhs_iface, void ** lhs_data);
+            std::byte * alloc_copy(void * src) noexcept;
+
+        private:
+            /** object storage **/
+            GCObjectStore * p_gco_store_ = nullptr;
+            /** collecting generations up to this bound **/
+            Generation upto_;
+        };
+
+    } /*namespace mm*/
+} /*namespace xo*/
+
+/* end DGCObjectVisitor.hpp */