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>

.gitignore

@@ -1,6 +1,7 @@
 CMakeFiles/
-build/
+build*/
 .cache/
+CMakeUserPresets.json
 build_warnings*.log
 final_build.log
 cmake_configure.log

CLAUDE.md

@@ -12,7 +12,7 @@ export MAMBA_ROOT_PREFIX="/home/share/micromamba"
 eval "$(/home/share/micromamba/bin/micromamba shell hook --shell bash)"
 micromamba activate mutom
 
-# Configure (from repo root, using Conan preset)
+# Configure (from repo root, using Conan preset — uses Ninja + ccache)
 cmake --preset conan-release
 
 # Build everything
@@ -40,6 +40,18 @@ conan install . --output-folder=build --build=missing
 cmake --preset conan-release
 ```
 
+### Build acceleration (already configured)
+- **Ninja** generator — used automatically via the conan default profile (`~/.conan2/profiles/default`)
+- **ccache** — enabled via `CMAKE_CXX_COMPILER_LAUNCHER=ccache`; cached rebuilds are nearly instant (~0.3s vs ~25s cold)
+- **Clang 22 + lld** profile available (`~/.conan2/profiles/fast`) but blocked by template overload ambiguities in `src/Core/Archives.h` that need fixing for full compatibility
+
+To reconfigure with the fast profile once Archives.h is fixed:
+```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)
+```
+
 ## Architecture
 
 **uLib** is a C++ framework for Cosmic Muon Tomography (CMT), structured as layered shared libraries:

CMakePresets.json

@@ -12,6 +12,22 @@
                 "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"
+            }
+        },
         {
             "name": "mutom",
             "description": "",

CMakeUserPresets.json

@@ -1,9 +0,0 @@
-{
-    "version": 4,
-    "vendor": {
-        "conan": {}
-    },
-    "include": [
-        "build/CMakePresets.json"
-    ]
-}

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

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) {