[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/stress/many_read.cu

@@ -1,70 +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.cuh>
-
-#include <chrono>
-
-using namespace std::chrono;
-using namespace cuda::experimental::stf;
-
-/* wall-clock time */
-double gettime()
-{
-  auto now = system_clock::now().time_since_epoch();
-  return duration_cast<duration<double>>(now).count();
-}
-
-int main(int argc, char** argv)
-{
-  stream_ctx ctx;
-  const size_t N = 16;
-  double X[N], Y[N];
-
-  for (size_t i = 0; i < N; i++)
-  {
-    X[i] = 1.0;
-    Y[i] = 2.0;
-  }
-
-  auto lX = ctx.logical_data(X);
-  auto lY = ctx.logical_data(Y);
-
-  size_t nloops       = 10;
-  size_t inner_nloops = 1000;
-
-  if (argc > 1)
-  {
-    nloops = atol(argv[1]);
-  }
-
-  if (argc > 2)
-  {
-    inner_nloops = atol(argv[2]);
-  }
-
-  for (size_t j = 0; j < nloops; j++)
-  {
-    std::chrono::steady_clock::time_point start, stop;
-    start = std::chrono::steady_clock::now();
-
-    for (size_t i = 0; i < inner_nloops; i++)
-    {
-      ctx.task(lX.read(), lY.rw())->*[](cudaStream_t, auto, auto) {};
-    }
-
-    stop                                   = std::chrono::steady_clock::now();
-    std::chrono::duration<double> duration = stop - start;
-
-    fprintf(stderr, "Elapsed: %.2lf us per task\n", duration.count() * 1000000.0 / (inner_nloops));
-  }
-
-  ctx.finalize();
-}