[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/places/CMakeLists.txt

@@ -1,30 +0,0 @@
-set(places_example_sources thrust_device_data_place_allocator.cu)
-
-## cudax_add_places_example
-#
-# Add a places example executable and register it with ctest.
-#
-# target_name_var: Variable name to overwrite with the name of the example
-#   target. Useful for modifying the example/target after creation.
-# source: The source file for the example.
-#
-function(cudax_add_places_example target_name_var source)
-  get_filename_component(filename ${source} NAME_WE)
-
-  set(example_target cudax.example.places.${filename})
-
-  cccl_add_executable(${example_target} SOURCES ${source} ADD_CTEST)
-  cudax_places_configure_target(${example_target})
-  target_link_libraries(
-    ${example_target}
-    PRIVATE #
-      cudax.compiler_interface
-      cudax.examples.thrust
-  )
-
-  set(${target_name_var} ${example_target} PARENT_SCOPE)
-endfunction()
-
-foreach (source IN LISTS places_example_sources)
-  cudax_add_places_example(example_target "${source}")
-endforeach()