[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/cmake/cudaxPlacesConfigureTarget.cmake

@@ -1,23 +0,0 @@
-# Configures a target for the Places framework.
-function(cudax_places_configure_target target_name)
-  target_link_libraries(
-    ${target_name}
-    PRIVATE #
-      CUDA::cudart_static
-      CUDA::cuda_driver
-  )
-
-  target_compile_options(
-    ${target_name}
-    PRIVATE
-      $<$<COMPILE_LANG_AND_ID:CUDA,NVIDIA>:--extended-lambda>
-      $<$<COMPILE_LANG_AND_ID:CUDA,NVIDIA>:--expt-relaxed-constexpr>
-  )
-
-  set_target_properties(
-    ${target_name}
-    PROPERTIES #
-      CUDA_RUNTIME_LIBRARY Static
-      CUDA_SEPARABLE_COMPILATION ON
-  )
-endfunction()