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>

README.md

@@ -41,7 +41,11 @@ conda activate mutom
 
 ### Configure and Build
 
-1. **Configure Conan profile (if you haven't yet on your machine):**
+#### Standard build (GCC + Ninja + ccache)
+
+The default conan profile uses **Ninja** as the generator and **ccache** for compiler caching, dramatically speeding up incremental rebuilds.
+
+1. **Configure Conan profile (first time only):**
 ```bash
 conan profile detect
 ```
@@ -51,17 +55,39 @@ conan profile detect
 conan install . --output-folder=build --build=missing
 ```
 
-3. **Configure the project with CMake:**
+3. **Configure with CMake:**
 ```bash
 cmake --preset conan-release
 ```
-*(Alternatively: `cd build && cmake .. -DCMAKE_TOOLCHAIN_FILE=conan_toolchain.cmake -DCMAKE_BUILD_TYPE=Release`)*
 
-4. **Build the project:**
+4. **Build:**
 ```bash
-cmake --build build -j10
+cmake --build build -j$(nproc)
 ```
 
+#### LLVM/Clang build (clang + lld + ccache — fastest)
+
+A `fast` conan profile is provided that uses **clang**, **lld** (LLVM linker), and **ccache**. Install them into your environment first:
+
+```bash
+micromamba install -n mutom -y clang clangxx lld -c conda-forge
+```
+
+Then build using the `fast` profile:
+
+```bash
+conan install . --output-folder=build --build=missing --profile=fast
+cmake -B build -G Ninja \
+      -DCMAKE_TOOLCHAIN_FILE=build/conan_toolchain.cmake \
+      -DCMAKE_BUILD_TYPE=Release
+cmake --build build -j$(nproc)
+```
+
+The `fast` profile is defined at `~/.conan2/profiles/fast` and sets:
+- `CMAKE_C_COMPILER=clang` / `CMAKE_CXX_COMPILER=clang++`
+- `CMAKE_EXE_LINKER_FLAGS=-fuse-ld=lld`
+- `CMAKE_CXX_COMPILER_LAUNCHER=ccache`
+
 ### Make python package
 
 ```bash