switch to Ninja+ccache, add clang/lld fast build profile

- CMakePresets.json: add 'fast' preset (clang+lld+ccache)
- .gitignore: generalize build/ to build*/, add CMakeUserPresets.json
- CMakeUserPresets.json: untrack (conan-generated, now gitignored)
- src/Core/Archives.h: remove redundant 'using basic_xml_iarchive::load_override'
  in xml_iarchive; caused ambiguous overload with clang (diamond inheritance)
- src/Core/Object.cpp: remove invalid explicit instantiations of non-template
  virtual Object::serialize (GCC extension, clang rejects)
- README.md, CLAUDE.md: document GCC and LLVM/clang build workflows

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Core/Archives.h

@@ -424,8 +424,6 @@ public:
     }
   }
 
-  using basic_xml_iarchive::load_override;
-
   // Anything not an attribute should be a name value pair as nvp or hrp
   typedef boost::archive::detail::common_iarchive<Archive>
       detail_common_iarchive;
@@ -443,6 +441,9 @@ public:
   // class_name_type can't be handled here as it depends upon the
   // char type used by the stream.  So require the derived implementation.
   // derived in this case is xml_iarchive_impl or base ..
+  // Note: using base::load_override covers all basic_xml_iarchive overloads
+  // transitively, so a separate 'using basic_xml_iarchive::load_override'
+  // is redundant and creates ambiguity with clang.
   using base::load_override;
 
   void load_override(const char *str) {