[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/examples/CMakeLists.txt

@@ -1,56 +0,0 @@
-thrust_create_target(cudax.examples.thrust)
-
-function(cudax_add_example target_name_var example_src)
-  get_filename_component(example_name ${example_src} NAME_WE)
-
-  # The actual name of the test's target:
-  set(example_target cudax.example.${example_name})
-  set(${target_name_var} ${example_target} PARENT_SCOPE)
-
-  cccl_add_executable(${example_target} SOURCES "${example_src}" ADD_CTEST)
-  target_link_libraries(
-    ${example_target}
-    PRIVATE #
-      cudax.compiler_interface
-      cudax.examples.thrust
-  )
-  target_compile_options(
-    ${example_target}
-    PRIVATE
-      $<$<COMPILE_LANG_AND_ID:CUDA,NVIDIA>:--expt-relaxed-constexpr>
-      $<$<COMPILE_LANG_AND_ID:CUDA,NVIDIA>:--extended-lambda>
-  )
-  target_include_directories(
-    ${example_target}
-    PRIVATE "${CUB_SOURCE_DIR}/examples"
-  )
-endfunction()
-
-file(
-  GLOB example_srcs
-  RELATIVE "${cudax_SOURCE_DIR}/examples"
-  CONFIGURE_DEPENDS
-  *.cu
-  *.cpp
-)
-
-cccl_get_cudatoolkit()
-
-# Example requires pinned_memory_resource.
-if (CUDAToolkit_VERSION VERSION_LESS 12.9)
-  list(REMOVE_ITEM example_srcs async_buffer_add.cu cub_reduce.cu)
-endif()
-
-foreach (example_src IN LISTS example_srcs)
-  cudax_add_example(example_target "${example_src}")
-endforeach()
-
-# FIXME: Enable MSVC
-if (cudax_ENABLE_CUDASTF AND NOT "MSVC" STREQUAL "${CMAKE_CXX_COMPILER_ID}")
-  # STF examples are handled separately:
-  add_subdirectory(stf)
-endif()
-
-if (cudax_ENABLE_PLACES AND NOT "MSVC" STREQUAL "${CMAKE_CXX_COMPILER_ID}")
-  add_subdirectory(places)
-endif()