[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/utility/ensure_current_device.cu

@@ -1,51 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// Part of CUDA Experimental 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) 2024 NVIDIA CORPORATION & AFFILIATES.
-//
-//===----------------------------------------------------------------------===//
-
-#include <cuda/experimental/__stream/stream.cuh>
-#include <cuda/experimental/__utility/ensure_current_device.cuh>
-#include <cuda/experimental/launch.cuh>
-
-#include <utility.cuh>
-
-namespace driver = cuda::__driver;
-
-void recursive_check_device_setter(int id)
-{
-  int cudart_id;
-  cudax::__ensure_current_device setter(cuda::device_ref{id});
-  REQUIRE(test::count_driver_stack() == cuda::devices.size() - id);
-  auto ctx = driver::__ctxGetCurrent();
-  REQUIRE_CUDART(cudaGetDevice(&cudart_id));
-  REQUIRE(cudart_id == id);
-
-  if (id != 0)
-  {
-    recursive_check_device_setter(id - 1);
-
-    REQUIRE(test::count_driver_stack() == cuda::devices.size() - id);
-    REQUIRE(ctx == driver::__ctxGetCurrent());
-    REQUIRE_CUDART(cudaGetDevice(&cudart_id));
-    REQUIRE(cudart_id == id);
-  }
-}
-
-C2H_TEST("ensure current device", "[device]")
-{
-  test::empty_driver_stack();
-  // If possible use something different than REQUIRE_CUDART default 0
-  int target_device = static_cast<int>(cuda::devices.size() - 1);
-
-  SECTION("device setter")
-  {
-    recursive_check_device_setter(target_device);
-
-    REQUIRE(test::count_driver_stack() == 0);
-  }
-}