[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/test/common/testing.cuh

@@ -1,110 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// Part of CUDA Experimental in CUDA C++ Core Libraries,
-// under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-// SPDX-FileCopyrightText: Copyright (c) 2023 NVIDIA CORPORATION & AFFILIATES.
-//
-//===----------------------------------------------------------------------===//
-
-#ifndef __COMMON_TESTING_H__
-#define __COMMON_TESTING_H__
-
-#include <cuda/__cccl_config>
-#include <cuda/__driver/driver_api.h>
-#include <cuda/std/__exception/terminate.h>
-
-#include <nv/target>
-
-#include <exception> // IWYU pragma: keep
-#include <iostream>
-#include <sstream>
-
-#include <c2h/catch2_test_helper.h>
-
-namespace cuda::experimental::execution
-{
-}
-
-namespace cudax       = cuda::experimental; // NOLINT: misc-unused-alias-decls
-namespace cudax_async = cuda::experimental::execution; // NOLINT: misc-unused-alias-decls
-
-__host__ __device__ constexpr bool operator==(const dim3& lhs, const dim3& rhs) noexcept
-{
-  return (lhs.x == rhs.x) && (lhs.y == rhs.y) && (lhs.z == rhs.z);
-}
-
-namespace Catch
-{
-template <>
-struct StringMaker<dim3>
-{
-  static std::string convert(dim3 const& dims)
-  {
-    std::ostringstream oss;
-    oss << "(" << dims.x << ", " << dims.y << ", " << dims.z << ")";
-    return oss.str();
-  }
-};
-} // namespace Catch
-
-namespace
-{
-namespace test
-{
-inline int count_driver_stack()
-{
-  if (cuda::__driver::__ctxGetCurrent() != nullptr)
-  {
-    auto ctx    = cuda::__driver::__ctxPop();
-    auto result = 1 + count_driver_stack();
-    cuda::__driver::__ctxPush(ctx);
-    return result;
-  }
-  else
-  {
-    return 0;
-  }
-}
-
-inline void empty_driver_stack()
-{
-  while (cuda::__driver::__ctxGetCurrent() != nullptr)
-  {
-    cuda::__driver::__ctxPop();
-  }
-}
-
-inline int cuda_driver_version()
-{
-  return cuda::__driver::__getVersion();
-}
-
-// Needs to be a template because we use template catch2 macro
-template <typename Dummy = void>
-struct ccclrt_test_fixture
-{
-  ccclrt_test_fixture()
-  {
-    empty_driver_stack();
-  }
-  ~ccclrt_test_fixture()
-  {
-    CHECK(count_driver_stack() == 0);
-  }
-};
-} // namespace test
-} // namespace
-
-// Test macro that should be used in all cccl-rt tests
-// It first empties the driver stack in case some other test has left it non-empty
-// and then runs the test. At the end it checks if it remained empty, which ensures
-// we don't accidentally initialize device 0 through REQUIRE_CUDART usage and makes sure
-// our APIs work with empty driver stack.
-#define C2H_CCCLRT_TEST(NAME, TAGS, ...) C2H_TEST_WITH_FIXTURE(::test::ccclrt_test_fixture, NAME, TAGS, __VA_ARGS__)
-
-#define C2H_CCCLRT_TEST_LIST(NAME, TAGS, ...) \
-  C2H_TEST_LIST_WITH_FIXTURE(::test::ccclrt_test_fixture, NAME, TAGS, __VA_ARGS__)
-
-#endif // __COMMON_TESTING_H__