xo-gc stack: refactor: introduce GCObjectVisitor facet

Plan using to properly level GCObjectStore and MutationLogStore
below Collector.

[WIP] not used yet

include/xo/alloc2/gc/IGCObjectVisitor_Any.hpp

@@ -0,0 +1,88 @@
+/** @file IGCObjectVisitor_Any.hpp
+ *
+ *  Generated automagically from ingredients:
+ *  1. code generator:
+ *       [xo-facet/codegen/genfacet]
+ *     arguments:
+ *       --input [idl/GCObjectVisitor.json5]
+ *  2. jinja2 template for abstract facet .hpp file:
+ *       [iface_facet_any.hpp.j2]
+ *  3. idl for facet methods
+ *       [idl/GCObjectVisitor.json5]
+ **/
+
+#pragma once
+
+#include "AGCObjectVisitor.hpp"
+#include <xo/facet/obj.hpp>
+
+namespace xo { namespace mm { class IGCObjectVisitor_Any; } }
+
+namespace xo {
+namespace facet {
+
+template <>
+struct FacetImplementation<xo::mm::AGCObjectVisitor,
+                           DVariantPlaceholder>
+{
+    using ImplType = xo::mm::IGCObjectVisitor_Any;
+};
+
+}
+}
+
+namespace xo {
+namespace mm {
+
+    /** @class IGCObjectVisitor_Any
+     *  @brief AGCObjectVisitor implementation for empty variant instance
+     **/
+    class IGCObjectVisitor_Any : public AGCObjectVisitor {
+    public:
+        /** @defgroup mm-gcobjectvisitor-any-type-traits **/
+        ///@{
+
+        /** integer identifying a type **/
+        using typeseq = xo::facet::typeseq;
+
+        ///@}
+        /** @defgroup mm-gcobjectvisitor-any-methods **/
+        ///@{
+
+        const AGCObjectVisitor * iface() const { return std::launder(this); }
+
+        // from AGCObjectVisitor
+
+        // builtin methods
+        typeseq _typeseq() const noexcept override { return s_typeseq; }
+        [[noreturn]] void _drop(Opaque) const noexcept override { _fatal(); }
+
+        // const methods
+
+        // nonconst methods
+        [[noreturn]] void visit_child(Opaque, AGCObject *, void **)  const  noexcept override;
+
+        ///@}
+
+    private:
+        /** @defgraoup mm-gcobjectvisitor-any-private-methods **/
+        ///@{
+
+        [[noreturn]] static void _fatal();
+
+        ///@}
+
+    public:
+        /** @defgroup mm-gcobjectvisitor-any-member-vars **/
+        ///@{
+
+        static typeseq s_typeseq;
+        static bool _valid;
+
+        ///@}
+    };
+
+} /*namespace mm */
+} /*namespace xo */
+
+/* IGCObjectVisitor_Any.hpp */