[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/graph/epoch.cu

@@ -1,56 +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 Test explicit uses of the API to change stage and create a sequence
- *        of CUDA graphs
- */
-
-#include <cuda/experimental/__stf/graph/graph_ctx.cuh>
-
-using namespace cuda::experimental::stf;
-
-int main()
-{
-  graph_ctx ctx;
-
-  const size_t N     = 8;
-  const size_t NITER = 2;
-
-  double A[N];
-  for (size_t i = 0; i < N; i++)
-  {
-    A[i] = 1.0 * i;
-  }
-
-  auto lA = ctx.logical_data(A);
-
-  for (size_t k = 0; k < NITER; k++)
-  {
-    ctx.parallel_for(blocked_partition(), exec_place::current_device(), lA.shape(), lA.rw())
-        ->*[] __host__ __device__(size_t i, slice<double> A) { A(i) = cos(A(i)); };
-
-    ctx.change_stage();
-  }
-
-  ctx.finalize();
-
-  for (size_t i = 0; i < N; i++)
-  {
-    double Ai_ref = 1.0 * i;
-    for (size_t k = 0; k < NITER; k++)
-    {
-      Ai_ref = cos(Ai_ref);
-    }
-
-    EXPECT(fabs(A[i] - Ai_ref) < 0.01);
-  }
-}