[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/pi.cu

@@ -1,52 +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 Approximate pi using Monte Carlo method
- *
- */
-
-#include <cuda/experimental/stf.cuh>
-
-#include <curand_kernel.h>
-#include <stdio.h>
-
-using namespace cuda::experimental::stf;
-
-int main(int, char**)
-{
-  context ctx;
-  auto lsum = ctx.logical_data(shape_of<scalar_view<size_t>>());
-
-  size_t N = 1000000;
-
-  ctx.parallel_for(box(N), lsum.reduce(reducer::sum<size_t>{}))->*[] __device__(size_t i, auto& sum) {
-    curandState local_state;
-    curand_init(1234, i, 0, &local_state);
-    double x = curand_uniform_double(&local_state); // Random x in [0, 1)
-    double y = curand_uniform_double(&local_state); // Random y in [0, 1)
-    // Count (x,y) coordinates which are within the unit circle
-    if (x * x + y * y <= 1.0)
-    {
-      sum++;
-    }
-  };
-
-  // We get the ratio of "shots" within the unit circle and the total number of
-  // "shots". The surface of the quarter of unit circle [0, 1) x [0, 1) is pi/4
-  auto res      = ctx.wait(lsum);
-  double pi_val = (4.0 * res) / N;
-
-  ctx.finalize();
-
-  _CCCL_ASSERT(fabs(pi_val - 3.1415) < 0.1, "Invalid result");
-}