[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/algorithm/fill.cu

@@ -1,93 +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 "common.cuh"
-
-C2H_TEST("Fill", "[data_manipulation]")
-{
-  cuda::stream _stream{cuda::device_ref{0}};
-  SECTION("Host resource")
-  {
-    cuda::mr::legacy_pinned_memory_resource host_resource;
-    cudax::uninitialized_buffer<int, cuda::mr::device_accessible> buffer(host_resource, buffer_size);
-
-    cuda::fill_bytes(_stream, buffer, fill_byte);
-
-    check_result_and_erase(_stream, cuda::std::span(buffer));
-  }
-
-  SECTION("Device resource")
-  {
-    cuda::device_memory_pool_ref device_resource = cuda::device_default_memory_pool(cuda::device_ref{0});
-    cudax::uninitialized_buffer<int, cuda::mr::device_accessible> buffer(device_resource, buffer_size);
-    cuda::fill_bytes(_stream, buffer, fill_byte);
-
-    std::vector<int> host_vector(42);
-    REQUIRE_CUDART(cudaMemcpyAsync(
-      host_vector.data(), buffer.data(), buffer.size() * sizeof(int), cudaMemcpyDefault, _stream.get()));
-
-    check_result_and_erase(_stream, host_vector);
-  }
-  SECTION("Launch transform")
-  {
-    cuda::mr::legacy_pinned_memory_resource host_resource;
-    cudax::weird_buffer buffer(host_resource, buffer_size);
-
-    cuda::fill_bytes(_stream, buffer, fill_byte);
-    check_result_and_erase(_stream, cuda::std::span(buffer.data, buffer.size));
-  }
-}
-
-C2H_TEST("Mdspan Fill", "[data_manipulation]")
-{
-  cuda::stream stream{cuda::device_ref{0}};
-  {
-    cuda::std::dextents<size_t, 3> dynamic_extents{1, 2, 3};
-    auto buffer = make_buffer_for_mdspan(dynamic_extents, 0);
-    cuda::std::mdspan<int, decltype(dynamic_extents)> dynamic_mdspan(buffer.data(), dynamic_extents);
-
-    cuda::fill_bytes(stream, dynamic_mdspan, fill_byte);
-    check_result_and_erase(stream, cuda::std::span(buffer.data(), buffer.size()));
-  }
-  {
-    cuda::std::extents<size_t, 2, cuda::std::dynamic_extent, 4> mixed_extents{1};
-    auto buffer = make_buffer_for_mdspan(mixed_extents, 0);
-    cuda::std::mdspan<int, decltype(mixed_extents)> mixed_mdspan(buffer.data(), mixed_extents);
-
-    cuda::fill_bytes(stream, cuda::std::move(mixed_mdspan), fill_byte);
-    check_result_and_erase(stream, cuda::std::span(buffer.data(), buffer.size()));
-  }
-  {
-    cuda::mr::legacy_pinned_memory_resource host_resource;
-    using static_extents = cuda::std::extents<size_t, 2, 3, 4>;
-    auto size            = cuda::std::layout_left::mapping<static_extents>().required_span_size();
-    cudax::weird_buffer<cuda::std::mdspan<int, static_extents>> buffer(host_resource, size);
-
-    cuda::fill_bytes(stream, buffer, fill_byte);
-    check_result_and_erase(stream, cuda::std::span(buffer.data, buffer.size));
-  }
-}
-
-C2H_TEST("Non exhaustive mdspan fill_bytes", "[data_manipulation]")
-{
-  cuda::stream stream{cuda::device_ref{0}};
-  {
-    auto fake_strided_mdspan = create_fake_strided_mdspan();
-
-    try
-    {
-      cuda::fill_bytes(stream, fake_strided_mdspan, fill_byte);
-    }
-    catch (const ::std::invalid_argument& e)
-    {
-      CHECK(e.what() == ::std::string("fill_bytes supports only exhaustive mdspans"));
-    }
-  }
-}