[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/cudaxBuildCompilerTargets.cmake

@@ -1,65 +0,0 @@
-# Including this file defines the following targets:
-#
-# cudax.compiler_interface
-# - Interface target that includes all compiler settings for cudax tests, etc.
-
-cccl_get_cub()
-cccl_get_cudax()
-cccl_get_libcudacxx()
-cccl_get_thrust()
-
-set(cuda_compile_options)
-set(cxx_compile_options)
-set(cxx_compile_definitions)
-
-if ("MSVC" STREQUAL "${CMAKE_CXX_COMPILER_ID}")
-  # C4848: support for attribute 'msvc::no_unique_address' in C++17 and earlier is a vendor extension
-  append_option_if_available("/wd4848" cxx_compile_options)
-
-  # XXX Temporary hack for STF !
-  # C4267: conversion from 'meow' to 'purr', possible loss of data
-  append_option_if_available("/wd4267" cxx_compile_options)
-
-  # C4459 : declaration of 'identifier' hides global declaration
-  # We work around std::chrono::last which hides some internal "last" variable
-  append_option_if_available("/wd4459" cxx_compile_options)
-
-  # stf used getenv which is potentially unsafe but not in our context
-  list(APPEND cxx_compile_definitions "_CRT_SECURE_NO_WARNINGS")
-endif()
-
-if ("Clang" STREQUAL "${CMAKE_CXX_COMPILER_ID}")
-  # stf heavily uses host device lambdas which break on clang due to a warning about the implicitly
-  # deleted copy constructor
-  # TODO(bgruber): remove this when NVBug 4980157 is resolved
-  append_option_if_available("-Wno-deprecated-copy" cxx_compile_options)
-endif()
-
-list(APPEND cxx_compile_definitions CCCL_ENABLE_ASSERTIONS)
-
-# Some groups related experimental code is located directly in libcu++ and is guarded by
-# _CUDAX_ENABLE_GROUP_FEATURES_IN_LIBCUDACXX macro, otherwise it would lead to a lot of code duplication. We define this
-# macro for cudax code globally, to get access to get access to the code.
-#
-# Can be removed once groups are no longer experimental.
-list(APPEND cxx_compile_definitions _CUDAX_ENABLE_GROUP_FEATURES_IN_LIBCUDACXX)
-
-cccl_build_compiler_interface(
-  cudax.compiler_flags
-  "${cuda_compile_options}"
-  "${cxx_compile_options}"
-  "${cxx_compile_definitions}"
-)
-
-add_library(cudax.compiler_interface INTERFACE)
-target_link_libraries(
-  cudax.compiler_interface
-  INTERFACE
-    # order matters here, we need the cudax options to override the cccl options.
-    cccl.compiler_interface
-    cudax.compiler_flags
-    libcudacxx::libcudacxx
-    CUB::CUB
-    Thrust::Thrust
-    cudax::cudax
-)