[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/utility.cuh

@@ -1,151 +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) 2024 NVIDIA CORPORATION & AFFILIATES.
-//
-//===----------------------------------------------------------------------===//
-
-#ifndef __COMMON_UTILITY_H__
-#define __COMMON_UTILITY_H__
-
-#include <cuda_runtime_api.h>
-// cuda_runtime_api needs to come first
-
-#include <cuda/__runtime/api_wrapper.h>
-#include <cuda/__stream/stream_ref.h>
-#include <cuda/atomic>
-#include <cuda/std/utility>
-
-#include <cuda/experimental/launch.cuh>
-
-#include <new> // IWYU pragma: keep (needed for placement new)
-
-#include "testing.cuh"
-
-namespace
-{
-namespace test
-{
-constexpr auto one_thread_dims = cuda::make_config(cuda::block_dims<1>(), cuda::grid_dims<1>());
-
-struct _malloc_pinned
-{
-private:
-  void* pv = nullptr;
-
-public:
-  explicit _malloc_pinned(std::size_t size)
-  {
-    cuda::__ensure_current_context guard(cuda::device_ref{0});
-    _CCCL_TRY_CUDA_API(::cudaMallocHost, "failed to allocate pinned memory", &pv, size);
-  }
-
-  ~_malloc_pinned()
-  {
-    cuda::__ensure_current_context guard(cuda::device_ref{0});
-    [[maybe_unused]] auto status = ::cudaFreeHost(pv);
-  }
-
-  template <class T>
-  T* get_as() const noexcept
-  {
-    return static_cast<T*>(pv);
-  }
-};
-
-template <class T>
-struct pinned
-{
-private:
-  _malloc_pinned _mem;
-
-public:
-  explicit pinned(T t)
-      : _mem(sizeof(T))
-  {
-    ::new (_mem.get_as<void>()) T(std::move(t));
-  }
-
-  ~pinned()
-  {
-    get()->~T();
-  }
-
-  T* get() noexcept
-  {
-    return _mem.get_as<T>();
-  }
-  const T* get() const noexcept
-  {
-    return _mem.get_as<T>();
-  }
-
-  T& operator*() noexcept
-  {
-    return *get();
-  }
-  const T& operator*() const noexcept
-  {
-    return *get();
-  }
-};
-
-template <int N>
-struct assign_n
-{
-  __device__ constexpr void operator()(int* pi) const noexcept
-  {
-    *pi = N;
-  }
-};
-
-template <int N>
-struct verify_n
-{
-  __device__ void operator()(int* pi) const noexcept
-  {
-    REQUIRE_DEVICE(*pi == N);
-  }
-};
-
-using assign_42 = assign_n<42>;
-using verify_42 = verify_n<42>;
-
-struct atomic_add_one
-{
-  __device__ void operator()(int* pi) const noexcept
-  {
-    cuda::atomic_ref atomic_pi(*pi);
-    atomic_pi.fetch_add(1);
-  }
-};
-
-struct atomic_sub_one
-{
-  __device__ void operator()(int* pi) const noexcept
-  {
-    cuda::atomic_ref atomic_pi(*pi);
-    atomic_pi.fetch_sub(1);
-  }
-};
-
-struct spin_until_80
-{
-  __device__ void operator()(int* pi) const noexcept
-  {
-    cuda::atomic_ref atomic_pi(*pi);
-    while (atomic_pi.load() != 80)
-      ;
-  }
-};
-
-struct empty_kernel
-{
-  __device__ void operator()() const noexcept {}
-};
-} // namespace test
-} // namespace
-#endif // __COMMON_UTILITY_H__