[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/dot/sections_2.cu

@@ -1,49 +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 This test makes sure we can generate a dot file with sections
- */
-
-#include <cuda/experimental/stf.cuh>
-
-using namespace cuda::experimental::stf;
-
-int main()
-{
-// TODO (miscco): Make it work for windows
-#if !_CCCL_COMPILER(MSVC)
-  context ctx;
-  auto lA = ctx.token().set_symbol("A");
-  auto lB = ctx.token().set_symbol("B");
-  auto lC = ctx.token().set_symbol("C");
-
-  // Begin a top-level section named "foo"
-  auto s_foo = ctx.dot_section("foo");
-  for (size_t i = 0; i < 2; i++)
-  {
-    // Section named "bar" using RAII
-    auto s_bar = ctx.dot_section("bar");
-    ctx.task(lA.read(), lB.rw()).set_symbol("t1")->*[](cudaStream_t) {};
-    for (size_t j = 0; j < 2; j++)
-    {
-      // Section named "baz" using RAII
-      auto s_bar = ctx.dot_section("baz");
-      ctx.task(lA.read(), lC.rw()).set_symbol("t2")->*[](cudaStream_t) {};
-      ctx.task(lB.read(), lC.read(), lA.rw()).set_symbol("t3")->*[](cudaStream_t) {};
-      // Implicit end of section "baz"
-    }
-    // Implicit end of section "bar"
-  }
-  s_foo.end(); // Explicit end of section "foo"
-  ctx.finalize();
-#endif // !_CCCL_COMPILER(MSVC)
-}