[INFRA] Import NVIDIA/CCCL upstream as optimization reference library

CCCL (CUDA C++ Core Libraries) provides:
- CUB: device/block/warp-level GPU primitives (reduce, scan, sort, topk)
- Thrust: high-level parallel algorithms (transform_reduce, sort, scan)
- libcudacxx: CUDA C++ standard library (atomics, barriers, memory)
- cudax: experimental features (memory resources, allocators)
- Tuning policies: per-SM hardware-specific algorithm parameters

Competition optimization vectors mapped to CCCL:
- Output TPS (83% weight): warp_reduce, block_reduce, device_topk
- Input TPS (14% weight): device_scan, block_load, prefetch
- Cache TPS (3% weight): prefix caching strategy patterns
- Memory (0.9 util): pooled/cached/buddy allocators

Source: https://github.com/NVIDIA/cccl (shallow clone, HEAD only)
License: Apache-2.0

cccl_upstream/libcudacxx/CMakeLists.txt

@@ -0,0 +1,49 @@
+if (NOT CCCL_ENABLE_LIBCUDACXX)
+  include(cmake/libcudacxxAddSubdir.cmake)
+  return()
+endif()
+
+cmake_minimum_required(VERSION 3.21)
+set(PACKAGE_NAME libcudacxx)
+set(PACKAGE_VERSION 11.0)
+set(PACKAGE_STRING "${PACKAGE_NAME} ${PACKAGE_VERSION}")
+project(libcudacxx LANGUAGES CXX)
+
+# Add codegen module
+option(
+  libcudacxx_ENABLE_CODEGEN
+  "Enable libcudacxx's atomics backend codegen and tests."
+  OFF
+)
+if (libcudacxx_ENABLE_CODEGEN)
+  add_subdirectory(codegen)
+endif()
+
+list(PREPEND CMAKE_MODULE_PATH "${libcudacxx_SOURCE_DIR}/cmake")
+set(LLVM_PATH "${libcudacxx_SOURCE_DIR}" CACHE STRING "" FORCE)
+
+# Configuration options.
+option(LIBCUDACXX_ENABLE_CUDA "Enable the CUDA language support." ON)
+if (LIBCUDACXX_ENABLE_CUDA)
+  enable_language(CUDA)
+endif()
+
+option(LIBCUDACXX_ENABLE_LIBCUDACXX_TESTS "Enable libcu++ tests." ON)
+if (LIBCUDACXX_ENABLE_LIBCUDACXX_TESTS)
+  enable_testing()
+
+  # Create the compiler targets with the common compile flags
+  include(cmake/LibcudacxxBuildCompilerTargets.cmake)
+  libcudacxx_build_compiler_targets()
+
+  # Test all public and internal headers
+  include(cmake/LibcudacxxInternalHeaderTesting.cmake)
+  include(cmake/LibcudacxxPublicHeaderTesting.cmake)
+  include(cmake/LibcudacxxPublicHeaderTestingHost.cmake)
+
+  add_subdirectory(test)
+endif()
+
+if (CCCL_ENABLE_BENCHMARKS)
+  add_subdirectory(benchmarks)
+endif()