[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/examples/05-stencil2d-places.cu

@@ -1,114 +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/__places/partitions/tiled_partition.cuh>
-#include <cuda/experimental/__stf/stream/stream_ctx.cuh>
-#include <cuda/experimental/__stf/utility/pretty_print.cuh>
-
-using namespace cuda::experimental::stf;
-
-template <typename T>
-__global__ void stencil2D_kernel(slice<T, 2> sUn, slice<const T, 2> sUn1)
-{
-  size_t N = sUn.extent(0);
-  for (size_t i = threadIdx.x + blockIdx.x * blockDim.x; i < N; i += blockDim.x * gridDim.x)
-  {
-    for (size_t j = 0; j < N; j++)
-    {
-      sUn(j, i) = 0.8 * sUn1(j, i) + 0.05 * sUn1(j, (i + 1) % N) + 0.05 * sUn1(j, (i - 1 + N) % N)
-                + 0.05 * sUn1((j + 1) % N, i) + 0.05 * sUn1((j - 1 + N) % N, i);
-    }
-  }
-}
-
-int main(int argc, char** argv)
-{
-  stream_ctx ctx;
-
-  size_t NITER  = 500;
-  size_t N      = 1000;
-  bool vtk_dump = false;
-
-  if (argc > 1)
-  {
-    NITER = atoi(argv[1]);
-  }
-
-  if (argc > 2)
-  {
-    N = atoi(argv[2]);
-  }
-
-  if (argc > 3)
-  {
-    int val  = atoi(argv[3]);
-    vtk_dump = (val == 1);
-  }
-
-  size_t TOTAL_SIZE = N * N;
-
-  double* Un  = new double[TOTAL_SIZE];
-  double* Un1 = new double[TOTAL_SIZE];
-
-  for (size_t idx = 0; idx < TOTAL_SIZE; idx++)
-  {
-    Un[idx]  = (idx == 0) ? 1.0 : 0.0;
-    Un1[idx] = Un[idx];
-  }
-
-  auto lUn  = ctx.logical_data(make_slice(Un, std::tuple{N, N}, N));
-  auto lUn1 = ctx.logical_data(make_slice(Un1, std::tuple{N, N}, N));
-
-  //    std::shared_ptr<execution_grid> all_devs = exec_place::all_devices();
-  // use grid [ 0 0 0 0 ] for debugging purpose
-  auto all_devs = exec_place::repeat(exec_place::device(0), 4);
-
-  // Partition over the vector of processor along the y-axis of the data domain
-  // TODO implement the proper tiled_partitioning along y !
-  data_place cdp = data_place::composite(tiled_partition<128>(), all_devs);
-
-  for (size_t iter = 0; iter < NITER; iter++)
-  {
-    // UPDATE Un from Un1
-    ctx.task(lUn.rw(cdp), lUn1.read(cdp))->*[&](auto stream, auto sUn, auto sUn1) {
-      stencil2D_kernel<double><<<32, 128, 0, stream>>>(sUn, sUn1);
-    };
-
-    // We make sure that the total sum of elements remains constant
-    if (iter % 250 == 0)
-    {
-      double sum = 0.0;
-
-      ctx.task(exec_place::host(), lUn.read())->*[&](auto stream, auto sUn) {
-        cuda_safe_call(cudaStreamSynchronize(stream));
-        for (size_t j = 0; j < N; j++)
-        {
-          for (size_t i = 0; i < N; i++)
-          {
-            sum += sUn(j, i);
-          }
-        }
-
-        if (vtk_dump)
-        {
-          char str[32];
-          snprintf(str, 32, "Un_%05zu.vtk", iter);
-          mdspan_to_vtk(sUn, std::string(str));
-        }
-      };
-
-      // fprintf(stderr, "iter %d : CHECK SUM = %e\n", iter, sum);
-    }
-
-    std::swap(lUn, lUn1);
-  }
-
-  ctx.finalize();
-}