[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/reductions/slice2d_reduction.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/interfaces/slice_reduction_ops.cuh>
-#include <cuda/experimental/__stf/stream/stream_ctx.cuh>
-
-using namespace cuda::experimental::stf;
-
-__global__ void add(slice<int, 2> s, int val)
-{
-  size_t tid      = threadIdx.x + blockIdx.x * blockDim.x;
-  size_t nthreads = blockDim.x * gridDim.x;
-
-  for (size_t j = 0; j < s.extent(1); j++)
-  {
-    for (size_t i = tid; i < s.extent(0); i += nthreads)
-    {
-      s(i, j) += val;
-    }
-  }
-}
-
-int main()
-{
-  stream_ctx ctx;
-
-  int array[6] = {0, 1, 2, 3, 4, 5};
-
-  // auto handle = ctx.logical_data(slice<int, 2>(&array[0], std::tuple{ 2, 3 }, 2));
-  auto handle = ctx.logical_data(make_slice(&array[0], std::tuple{2, 3}, 2));
-
-  auto redux_op = std::make_shared<slice_reduction_op_sum<int, 2>>();
-
-  ctx.task(handle.relaxed(redux_op))->*[](auto stream, auto s) {
-    add<<<32, 32, 0, stream>>>(s, 42);
-  };
-
-  ctx.task(exec_place::host(), handle.read())->*[](auto stream, auto s) {
-    cuda_safe_call(cudaStreamSynchronize(stream));
-
-    for (size_t j = 0; j < s.extent(1); j++)
-    {
-      for (size_t i = 0; i < s.extent(0); i++)
-      {
-        // fprintf(stderr, "%d\t", s(i, j));
-      }
-      // fprintf(stderr, "\n");
-    }
-  };
-
-  ctx.finalize();
-}