[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/stf/slice/pinning.cu

@@ -1,59 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// Part of CUDASTF 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) 2022-2024 NVIDIA CORPORATION & AFFILIATES.
-//
-//===----------------------------------------------------------------------===//
-
-#include <cuda/experimental/__stf/stream/stream_ctx.cuh>
-
-#include <iostream>
-
-using namespace cuda::experimental::stf;
-
-int main()
-{
-  stream_ctx ctx;
-
-  // Contiguous 1D
-  double* X     = new double[1024];
-  auto handle_X = ctx.logical_data(make_slice(X, 1024));
-
-  // Contiguous 2D
-  double* X2     = new double[1024 * 1024];
-  auto handle_X2 = ctx.logical_data(make_slice(X2, std::tuple{1024, 1024}, 1024));
-
-  // Contiguous 3D
-  double* X4     = new double[128 * 128 * 128];
-  auto handle_X4 = ctx.logical_data(make_slice(X4, std::tuple{128, 128, 128}, 128, 128 * 128));
-
-  // Discontiguous 2D
-  double* X3     = new double[128 * 8];
-  auto handle_X3 = ctx.logical_data(make_slice(X3, std::tuple{64, 8}, 128));
-
-  // Discontiguous 3D
-  double* X5     = new double[32 * 4 * 4];
-  auto handle_X5 = ctx.logical_data(make_slice(X5, std::tuple{16, 4, 4}, 32, 32 * 4));
-
-  double* X6     = new double[32 * 4 * 4];
-  auto handle_X6 = ctx.logical_data(make_slice(X6, std::tuple{32, 2, 4}, 32, 32 * 4));
-
-  double* X7 = new double[128 * 128 * 128];
-  cuda_safe_call(cudaHostRegister(X7, 128 * 128 * 128, cudaHostRegisterPortable));
-  auto handle_X7 = ctx.logical_data(make_slice(X7, std::tuple{128, 128, 128}, 128, 128 * 128));
-
-  // Detect that this was already pinned
-  double* X9 = new double[1024];
-  cuda_safe_call(cudaHostRegister(X9, 1024, cudaHostRegisterPortable));
-  auto handle_X9 = ctx.logical_data(make_slice(X9, 1024));
-
-  // Detect that this was already pinned
-  double* X8 = new double[4 * 4 * 4];
-  cuda_safe_call(cudaHostRegister(X8, 4 * 4 * 4, cudaHostRegisterPortable));
-  auto handle_X8 = ctx.logical_data(make_slice(X8, std::tuple{1, 4, 4}, 4, 4 * 4));
-
-  ctx.finalize();
-}