[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/cudaxHeaderTesting.cmake

@@ -1,109 +0,0 @@
-# For every public header, build a translation unit containing `#include <header>`
-# to let the compiler try to figure out warnings in that header if it is not otherwise
-# included in tests, and also to verify if the headers are modular enough.
-# .inl files are not globbed for, because they are not supposed to be used as public
-# entrypoints.
-
-cccl_get_cudatoolkit()
-
-# Meta target for all configs' header builds:
-add_custom_target(cudax.all.headers)
-
-function(cudax_add_header_test label definitions)
-  ###################
-  # Non-STF headers #
-  set(headertest_target cudax.headers.${label}.no_stf)
-  cccl_generate_header_tests(
-    ${headertest_target}
-    cudax/include
-    # The cudax header template removes the check for the `small` macro.
-    HEADER_TEMPLATE "${cudax_SOURCE_DIR}/cmake/header_test.in.cu"
-    GLOBS "cuda/experimental/*.cuh"
-    EXCLUDES
-      # The following internal headers are not required to compile independently:
-      "cuda/experimental/__execution/prologue.cuh"
-      "cuda/experimental/__execution/epilogue.cuh"
-      # cuFile headers are compiled separately:
-      "cuda/experimental/cufile.cuh"
-      "cuda/experimental/__cufile/*"
-      # Places headers are compiled separately:
-      "cuda/experimental/places.cuh"
-      "cuda/experimental/__places/*"
-      # STF headers are compiled separately:
-      "cuda/experimental/stf.cuh"
-      "cuda/experimental/__stf/*"
-  )
-  target_link_libraries(${headertest_target} PUBLIC cudax.compiler_interface)
-
-  if (cudax_ENABLE_CUFILE)
-    ###############
-    # cuFile headers #
-    set(headertest_target cudax.headers.${label}.cufile)
-    cccl_generate_header_tests(
-      ${headertest_target}
-      cudax/include
-      HEADER_TEMPLATE "${cudax_SOURCE_DIR}/cmake/header_test.in.cu"
-      GLOBS #
-        "cuda/experimental/cufile.cuh"
-        "cuda/experimental/__cufile/*.cuh"
-    )
-    target_link_libraries(${headertest_target} PUBLIC cudax.compiler_interface)
-  endif()
-
-  # FIXME: Enable MSVC
-  if (cudax_ENABLE_PLACES AND NOT "MSVC" STREQUAL "${CMAKE_CXX_COMPILER_ID}")
-    ##################
-    # Places headers #
-    set(headertest_target cudax.headers.${label}.places)
-    cccl_generate_header_tests(
-      ${headertest_target}
-      cudax/include
-      GLOBS #
-        "cuda/experimental/places.cuh"
-        "cuda/experimental/__places/*.cuh"
-      HEADER_TEMPLATE "${cudax_SOURCE_DIR}/cmake/header_test.in.cu"
-    )
-    target_link_libraries(${headertest_target} PUBLIC cudax.compiler_interface)
-    target_compile_options(
-      ${headertest_target}
-      PRIVATE
-        $<$<COMPILE_LANG_AND_ID:CUDA,NVIDIA>:--extended-lambda>
-        $<$<COMPILE_LANG_AND_ID:CUDA,NVIDIA>:--expt-relaxed-constexpr>
-    )
-  endif()
-
-  # FIXME: Enable MSVC
-  if (cudax_ENABLE_CUDASTF AND NOT "MSVC" STREQUAL "${CMAKE_CXX_COMPILER_ID}")
-    ###############
-    # STF headers #
-    set(headertest_target cudax.headers.${label}.stf)
-    cccl_generate_header_tests(
-      ${headertest_target}
-      cudax/include
-      GLOBS #
-        "cuda/experimental/stf.cuh"
-        "cuda/experimental/__stf/*.cuh"
-      # FIXME: The cudax header template removes the check for the `small` macro.
-      # cuda/experimental/__stf/utility/memory.cuh defines functions named `small`.
-      # These should be renamed to avoid conflicts with windows system headers, and
-      # the following line removed:
-      HEADER_TEMPLATE "${cudax_SOURCE_DIR}/cmake/header_test.in.cu"
-    )
-    target_link_libraries(
-      ${headertest_target}
-      PUBLIC cudax.compiler_interface CUDA::cuda_driver
-    )
-    target_compile_options(
-      ${headertest_target}
-      PRIVATE
-        # Required by stf headers:
-        $<$<COMPILE_LANG_AND_ID:CUDA,NVIDIA>:--extended-lambda>
-        # FIXME: We should be able to refactor away from needing this by
-        # using _CCCL_HOST_DEVICE and friends + `::cuda::std` utilities where
-        # necessary.
-        $<$<COMPILE_LANG_AND_ID:CUDA,NVIDIA>:--expt-relaxed-constexpr>
-    )
-  endif()
-endfunction()
-
-cudax_add_header_test(basic "")