[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/examples/cub_reduce.cu

@@ -1,53 +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) 2025 NVIDIA CORPORATION & AFFILIATES.
-//
-//===----------------------------------------------------------------------===//
-
-// Example of using `cub::DeviceReduce::Reduce` with cudax environment.
-
-#include <cub/device/device_reduce.cuh>
-
-#include <cuda/experimental/container.cuh>
-#include <cuda/experimental/memory_resource.cuh>
-#include <cuda/experimental/stream.cuh>
-
-#include <iostream>
-
-namespace cudax = cuda::experimental;
-
-int main()
-{
-  constexpr int num_items = 50000;
-
-  // A CUDA stream on which to execute the reduction
-  cuda::stream stream{cuda::devices[0]};
-  cuda::device_memory_pool_ref mr = cuda::device_default_memory_pool(cuda::devices[0]);
-
-  // Allocate input and output, but do not zero initialize output (`cuda::no_init`)
-  auto d_in  = cuda::make_buffer<int>(stream, mr, num_items, 1);
-  auto d_out = cuda::make_buffer<float>(stream, mr, 1, cuda::no_init);
-
-  // An environment we use to pass all necessary information to CUB
-  cudax::env_t<cuda::mr::device_accessible> env{mr, stream};
-  auto error = cub::DeviceReduce::Reduce(d_in.begin(), d_out.begin(), num_items, cuda::std::plus{}, 0, env);
-  if (error != cudaSuccess)
-  {
-    std::cerr << "cub::DeviceReduce::Reduce failed: " << cudaGetErrorString(error) << "\n";
-    exit(EXIT_FAILURE);
-  }
-
-  auto h_out = cuda::make_buffer<float>(stream, cuda::pinned_default_memory_pool(), d_out);
-
-  stream.sync();
-
-  if (h_out.get_unsynchronized(0) != num_items)
-  {
-    std::cerr << "Result verification failed: " << h_out.get_unsynchronized(0) << " != " << num_items << "\n";
-    exit(EXIT_FAILURE);
-  }
-}