[CCCL] 瘦身 + 补全: 移除 cudax/python/libcudacxx-tests 冗余文件, 新增 c2h 测试助手 + cmake 构建系统 + 8 个 CUDA thrust examples

变更摘要:
- 删除: cudax/ (783 files, 7.2M) — 实验性组件，竞赛不需要
- 删除: python/ (226 files, 2.0M) — Python 绑定，竞赛不需要
- 删除: libcudacxx/{test,benchmarks,codegen,cmake,share} (4432 files, 31M)
  保留: libcudacxx/include/ (1463 headers, cuda::std 编译依赖)
- 新增: c2h/ (27 files) — CUB Catch2 测试辅助头文件，编译 243 个测试必需
- 新增: cmake/ (29 files) — CCCL 原生 CMake 构建系统
- 新增: thrust/examples/cuda/ (7 files) + cpp_integration/ (1 file)
  async_reduce, custom_temporary_allocation, explicit_cuda_stream,
  global_device_vector, range_view, unwrap_pointer, wrap_pointer, device

结果: cccl_upstream 从 74M→35M (瘦身 53%), 核心内容 100% 保留:
  27/27 tuning headers, 78 benchmarks, 243 tests,
  60 thrust examples, 18 CUB examples, 全部编译头文件

cccl_upstream/cudax/CMakeLists.txt

@@ -1,94 +0,0 @@
-if (NOT CCCL_ENABLE_CUDAX)
-  include(cmake/cudaxAddSubdir.cmake)
-  return()
-endif()
-
-cmake_minimum_required(VERSION 3.21)
-project(cudax LANGUAGES CXX CUDA)
-
-option(
-  cudax_ENABLE_HEADER_TESTING
-  "Test that CUDA Experimental's public headers compile."
-  ON
-)
-option(cudax_ENABLE_TESTING "Build CUDA Experimental's tests." ON)
-option(cudax_ENABLE_EXAMPLES "Build CUDA Experimental's examples." ON)
-option(cudax_ENABLE_PLACES "Enable standalone Places subproject" ON)
-option(cudax_ENABLE_CUDASTF "Enable CUDASTF subproject" ON)
-option(
-  cudax_ENABLE_CUDASTF_CODE_GENERATION
-  "Enable code generation using STF's parallel_for or launch with CUDA compiler."
-  ON
-)
-option(
-  cudax_ENABLE_CUDASTF_BOUNDSCHECK
-  "Enable bounds checks for STF targets. Requires debug build."
-  OFF
-)
-option(
-  cudax_ENABLE_CUDASTF_MATHLIBS
-  "Enable STF tests/examples that use cublas/cusolver."
-  OFF
-)
-option(cudax_ENABLE_CUFILE "Enable cuFile in CUDA Experimental" ON)
-
-if (cudax_ENABLE_CUFILE)
-  if (WIN32)
-    message(FATAL_ERROR "cuFile is not available on Windows.")
-  endif()
-
-  if (CMAKE_VERSION VERSION_LESS "3.25.0")
-    message(
-      FATAL_ERROR
-      "cuFile is not available before cmake 3.25.0, please, use newer cmake."
-    )
-  endif()
-
-  cccl_get_cudatoolkit()
-
-  if (CUDAToolkit_VERSION VERSION_LESS "12.9.0")
-    message(FATAL_ERROR "cuFile support requires at least CUDA 12.9.")
-  endif()
-
-  if (NOT TARGET CUDA::cuFile)
-    message(
-      FATAL_ERROR
-      "CUDA::cuFile target required for requested cuFile support was not found."
-    )
-  endif()
-endif()
-
-if (
-  cudax_ENABLE_CUDASTF_BOUNDSCHECK
-  AND NOT CMAKE_BUILD_TYPE MATCHES "Debug"
-  AND NOT CMAKE_BUILD_TYPE MATCHES "RelWithDebInfo"
-)
-  message(
-    FATAL_ERROR
-    "cudax_ENABLE_CUDASTF_BOUNDSCHECK requires a Debug build."
-  )
-endif()
-
-include(cmake/cudaxBuildCompilerTargets.cmake)
-if (cudax_ENABLE_PLACES)
-  include(cmake/cudaxPlacesConfigureTarget.cmake)
-endif()
-if (cudax_ENABLE_CUDASTF)
-  include(cmake/cudaxSTFConfigureTarget.cmake)
-endif()
-
-if (cudax_ENABLE_HEADER_TESTING)
-  include(cmake/cudaxHeaderTesting.cmake)
-endif()
-
-if (cudax_ENABLE_TESTING)
-  add_subdirectory(test)
-endif()
-
-if (cudax_ENABLE_EXAMPLES)
-  add_subdirectory(examples)
-endif()
-
-if (CCCL_ENABLE_BENCHMARKS)
-  add_subdirectory(benchmarks)
-endif()