[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/async_buffer_add.cu

@@ -1,92 +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.
-//
-//===----------------------------------------------------------------------===//
-
-/**
- * Vector addition: C = A + B.
- *
- * This sample is a very basic sample that implements element by element
- * vector addition. It is the same as the sample illustrating Chapter 2
- * of the programming guide with some additions like error checking.
- */
-
-#include <thrust/execution_policy.h>
-#include <thrust/random.h>
-#include <thrust/tabulate.h>
-#include <thrust/transform.h>
-
-#include <cuda/experimental/container.cuh>
-#include <cuda/experimental/memory_resource.cuh>
-#include <cuda/experimental/stream.cuh>
-
-#include <iostream>
-
-namespace cudax = cuda::experimental;
-
-constexpr int numElements = 50000;
-
-struct generator
-{
-  thrust::default_random_engine gen{};
-  thrust::uniform_real_distribution<float> dist{-10.0f, 10.0f};
-
-  __host__ __device__ generator(const unsigned seed)
-      : gen{seed}
-  {}
-
-  __host__ __device__ float operator()(cuda::std::size_t idx) noexcept
-  {
-    gen.discard(idx);
-    return dist(gen);
-  }
-};
-
-int main()
-{
-  // A CUDA stream on which to execute the vector addition kernel
-  cudax::stream stream{cuda::device_ref{0}};
-
-  // The execution policy we want to use to run all work on the same stream
-  auto policy = thrust::cuda::par_nosync.on(stream.get());
-
-  cuda::device_memory_pool_ref device_resource = cuda::device_default_memory_pool(cuda::device_ref{0});
-
-  // Allocate the two inputs and output, but do not zero initialize via `cuda::no_init`
-  cuda::device_buffer<float> A{stream, device_resource, numElements, cuda::no_init};
-  cuda::device_buffer<float> B{stream, device_resource, numElements, cuda::no_init};
-  cuda::device_buffer<float> C{stream, device_resource, numElements, cuda::no_init};
-
-  // Fill both vectors on stream using a random number generator
-  thrust::tabulate(policy, A.begin(), A.end(), generator{42});
-  thrust::tabulate(policy, B.begin(), B.end(), generator{1337});
-
-  // Add the vectors together
-  thrust::transform(policy, A.begin(), A.end(), B.begin(), C.begin(), cuda::std::plus<>{});
-
-  cuda::pinned_memory_pool_ref pinned_resource = cuda::pinned_default_memory_pool();
-
-  // Verify that the result vector is correct, by copying it to host
-  cuda::host_buffer<float> h_A{stream, pinned_resource, A};
-  cuda::host_buffer<float> h_B{stream, pinned_resource, B};
-  cuda::host_buffer<float> h_C{stream, pinned_resource, C};
-
-  // Do not forget to sync afterwards
-  stream.sync();
-
-  for (int i = 0; i < numElements; ++i)
-  {
-    if (cuda::std::abs(h_A.get_unsynchronized(i) + h_B.get_unsynchronized(i) - h_C.get_unsynchronized(i)) > 1e-5)
-    {
-      std::cerr << "Result verification failed at element " << i << "\n";
-      exit(EXIT_FAILURE);
-    }
-  }
-
-  return 0;
-}