[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/stf/frozen_data_init.cu

@@ -1,53 +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.
-//
-//===----------------------------------------------------------------------===//
-
-/**
- * @file
- *
- * @brief Illustrate how we can use frozen data to initialize constant data
- *
- */
-
-#include <cuda/experimental/stf.cuh>
-
-using namespace cuda::experimental::stf;
-
-int main()
-{
-  context ctx;
-
-  /* Create a piece of data that can be use many times without further synchronizations */
-  auto buffer = ctx.logical_data(shape_of<slice<double, 2>>(128, 64)).set_symbol("buffer");
-  ctx.parallel_for(buffer.shape(), buffer.write())->*[] __device__(size_t i, size_t j, auto b) {
-    b(i, j) = sin(-1.0 * i) + cos(2.0 * j);
-  };
-
-  auto frozen_buffer = ctx.freeze(buffer);
-
-  auto h_buf = frozen_buffer.get(data_place::host()).first;
-  auto d_buf = frozen_buffer.get(data_place::current_device()).first;
-
-  cuda_safe_call(cudaStreamSynchronize(ctx.fence()));
-
-  auto lX = ctx.logical_data(buffer.shape()).set_symbol("X");
-  ctx.parallel_for(lX.shape(), lX.write()).set_symbol("X=buf")->*[d_buf] __device__(size_t i, size_t j, auto x) {
-    x(i, j) = d_buf(i, j);
-  };
-
-  ctx.parallel_for(exec_place::host(), lX.shape(), lX.read()).set_symbol("check buf")
-      ->*[h_buf](size_t i, size_t j, auto x) {
-            EXPECT(fabs(x(i, j) - h_buf(i, j)) < 0.0001);
-          };
-
-  // Make sure all tasks are done before unfreezing
-  frozen_buffer.unfreeze(ctx.fence());
-
-  ctx.finalize();
-}