[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/unstable_unique.cu

@@ -1,101 +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) 2022-2025 NVIDIA CORPORATION & AFFILIATES.
-//
-//===----------------------------------------------------------------------===//
-
-#include <cuda/std/functional>
-
-#include <cuda/experimental/__utility/unstable_unique.cuh>
-
-#include <algorithm>
-#include <iterator>
-#include <list>
-#include <vector>
-
-#include <c2h/catch2_test_helper.h>
-
-namespace cudax = cuda::experimental;
-
-TEST_CASE("unstable_unique empty range", "[utility]")
-{
-  std::vector<int> v;
-  auto new_end = cudax::unstable_unique(v.begin(), v.end());
-  REQUIRE(v.end() == new_end);
-}
-
-TEST_CASE("unstable_unique no duplicates", "[utility]")
-{
-  std::vector<int> v = {1, 2, 3, 4, 5};
-  auto new_end       = cudax::unstable_unique(v.begin(), v.end());
-  REQUIRE(v.end() == new_end);
-  REQUIRE(std::vector<int>({1, 2, 3, 4, 5}) == v);
-}
-
-TEST_CASE("unstable_unique leading duplicates", "[utility]")
-{
-  std::vector<int> v = {1, 1, 2, 3, 4, 5};
-  auto new_end       = cudax::unstable_unique(v.begin(), v.end());
-  REQUIRE(v.begin() + 5 == new_end);
-  REQUIRE(std::vector<int>({1, 5, 2, 3, 4, 5}) == v);
-}
-
-TEST_CASE("unstable_unique interleaved duplicates", "[utility]")
-{
-  std::vector<int> v = {1, 1, 2, 2, 3, 3, 4, 4, 5, 5};
-  auto new_end       = cudax::unstable_unique(v.begin(), v.end());
-  REQUIRE(v.begin() + 5 == new_end);
-  REQUIRE(std::vector<int>({1, 5, 2, 4, 3, 3, 4, 4, 5, 5}) == v);
-}
-
-TEST_CASE("unstable_unique all same", "[utility]")
-{
-  std::vector<int> v = {1, 1, 1, 1, 1};
-  auto new_end       = cudax::unstable_unique(v.begin(), v.end());
-  REQUIRE(1 + v.begin() == new_end);
-  REQUIRE(std::vector<int>({1, 1, 1, 1, 1}) == v);
-}
-
-TEST_CASE("unstable_unique trailing unique", "[utility]")
-{
-  std::vector<int> v = {1, 1, 1, 1, 1, 2};
-  auto new_end       = cudax::unstable_unique(v.begin(), v.end());
-  REQUIRE(v.begin() + 2 == new_end);
-  REQUIRE(std::vector<int>({1, 2, 1, 1, 1, 2}) == v);
-}
-
-TEST_CASE("unstable_unique with custom predicate", "[utility]")
-{
-  std::vector<int> v = {1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 3, 4, 5};
-  auto new_end       = cudax::unstable_unique(v.begin(), v.end(), cuda::std::equal_to<int>{});
-  REQUIRE(v.begin() + 5 == new_end);
-  REQUIRE(std::vector<int>{1, 5, 4, 3, 2, 1, 1, 1, 1, 2, 2, 2, 3, 4, 5} == v);
-}
-
-TEST_CASE("unstable_unique on bidirectional iterators (std::list)", "[utility]")
-{
-  // std::list has bidirectional (not random-access) iterators -- exercises the
-  // !=-based loop termination path.
-  std::list<int> l = {1, 1, 2, 2, 3, 3, 4, 4, 5, 5};
-  auto new_end     = cudax::unstable_unique(l.begin(), l.end());
-  REQUIRE(std::distance(l.begin(), new_end) == 5);
-
-  std::vector<int> deduped(l.begin(), new_end);
-  std::sort(deduped.begin(), deduped.end());
-  REQUIRE(std::vector<int>({1, 2, 3, 4, 5}) == deduped);
-}
-
-TEST_CASE("unstable_unique on bidirectional iterators with custom predicate", "[utility]")
-{
-  std::list<int> l = {1, 1, 1, 1, 1, 2, 2, 3};
-  auto new_end     = cudax::unstable_unique(l.begin(), l.end(), cuda::std::equal_to<int>{});
-  REQUIRE(std::distance(l.begin(), new_end) == 3);
-
-  std::vector<int> deduped(l.begin(), new_end);
-  std::sort(deduped.begin(), deduped.end());
-  REQUIRE(std::vector<int>({1, 2, 3}) == deduped);
-}