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>

CMakePresets.json

@@ -11,6 +11,22 @@
                 "CMAKE_BUILD_TYPE": "Debug",
                 "CMAKE_INSTALL_PREFIX": "${sourceDir}/out/install/${presetName}"
             }
+        },
+        {
+            "name": "fast",
+            "displayName": "Fast build: Ninja + clang + ccache",
+            "description": "Uses Ninja generator, clang/lld compiler, and ccache",
+            "generator": "Ninja",
+            "binaryDir": "${sourceDir}/build",
+            "cacheVariables": {
+                "CMAKE_BUILD_TYPE": "Release",
+                "CMAKE_C_COMPILER": "clang",
+                "CMAKE_CXX_COMPILER": "clang++",
+                "CMAKE_EXE_LINKER_FLAGS": "-fuse-ld=lld",
+                "CMAKE_SHARED_LINKER_FLAGS": "-fuse-ld=lld",
+                "CMAKE_CXX_COMPILER_LAUNCHER": "ccache",
+                "CMAKE_C_COMPILER_LAUNCHER": "ccache"
+            }
         }
     ]
-}
+}