[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/copy/copy_vectorize.cu

@@ -1,91 +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) 2026 NVIDIA CORPORATION & AFFILIATES.
-//
-//===----------------------------------------------------------------------===//
-
-#include "copy_common.cuh"
-
-// All layouts in this file have 48 contiguous elements that should coalesce into a (48):(1) layout
-// and generate 16B vectorized copies.
-
-static constexpr int N = 48;
-
-/***********************************************************************************************************************
- * 1D Vectorization Test
- **********************************************************************************************************************/
-
-// src: (48):(1)
-// dst: (48):(1)
-TEST_CASE("copy d2d vectorize 48:1", "[copy][d2d][vectorize][1d]")
-{
-  test_copy<layout_right>(make_iota<int>(N), N);
-}
-
-/***********************************************************************************************************************
- * 2D Vectorization Tests
- **********************************************************************************************************************/
-
-// src: (6,8):(8,1)
-// dst: (6,8):(8,1)
-TEST_CASE("copy d2d vectorize (6,8):(8,1)", "[copy][d2d][vectorize][2d]")
-{
-  test_copy<layout_right>(make_iota<int>(N), 6, 8);
-}
-
-// src: (8,6):(1,8)
-// dst: (8,6):(1,8)
-TEST_CASE("copy d2d vectorize (8,6):(1,8)", "[copy][d2d][vectorize][2d]")
-{
-  test_copy<layout_left>(make_iota<int>(N), 8, 6);
-}
-
-// src: (6,8):(1,6)
-// dst: (6,8):(1,6)
-TEST_CASE("copy d2d vectorize (6,8):(1,6)", "[copy][d2d][vectorize][2d]")
-{
-  test_copy<layout_left>(make_iota<int>(N), 6, 8);
-}
-
-// src: (8,6):(6,1)
-// dst: (8,6):(6,1)
-TEST_CASE("copy d2d vectorize (8,6):(6,1)", "[copy][d2d][vectorize][2d]")
-{
-  test_copy<layout_right>(make_iota<int>(N), 8, 6);
-}
-
-/***********************************************************************************************************************
- * 2D Non-Square Vectorization Tests
- **********************************************************************************************************************/
-
-// src: (3,16):(1,3)
-// dst: (3,16):(1,3)
-TEST_CASE("copy d2d vectorize (3,16):(1,3)", "[copy][d2d][vectorize][2d]")
-{
-  test_copy<layout_left>(make_iota<int>(N), 3, 16);
-}
-
-// src: (3,16):(16,1)
-// dst: (3,16):(16,1)
-TEST_CASE("copy d2d vectorize (3,16):(16,1)", "[copy][d2d][vectorize][2d]")
-{
-  test_copy<layout_right>(make_iota<int>(N), 3, 16);
-}
-
-// src: (16,3):(1,16)
-// dst: (16,3):(1,16)
-TEST_CASE("copy d2d vectorize (16,3):(1,16)", "[copy][d2d][vectorize][2d]")
-{
-  test_copy<layout_left>(make_iota<int>(N), 16, 3);
-}
-
-// src: (16,3):(3,1)
-// dst: (16,3):(3,1)
-TEST_CASE("copy d2d vectorize (16,3):(3,1)", "[copy][d2d][vectorize][2d]")
-{
-  test_copy<layout_right>(make_iota<int>(N), 16, 3);
-}