[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/execution/common/dummy_scheduler.cuh

@@ -1,107 +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.
-//
-//===----------------------------------------------------------------------===//
-
-#pragma once
-
-#include <cuda/__utility/immovable.h>
-
-#include <cuda/experimental/execution.cuh>
-
-#include "testing.cuh" // IWYU pragma: keep
-
-namespace ex = cuda::experimental::execution;
-
-namespace
-{
-namespace _dummy
-{
-template <class Domain>
-struct _attrs_t
-{
-  _CCCL_HOST_DEVICE constexpr auto query(ex::get_completion_scheduler_t<ex::set_value_t>) const noexcept;
-
-  _CCCL_HOST_DEVICE constexpr auto query(ex::get_completion_domain_t<ex::set_value_t>) const noexcept
-  {
-    return Domain{};
-  }
-};
-
-template <class Rcvr>
-struct _opstate_t : cuda::__immovable
-{
-  using operation_state_concept = ex::operation_state_t;
-
-  _CCCL_HOST_DEVICE constexpr _opstate_t(Rcvr rcvr) noexcept
-      : _rcvr(static_cast<Rcvr&&>(rcvr))
-  {}
-
-  _CCCL_HOST_DEVICE constexpr void start() noexcept
-  {
-    ex::set_value(static_cast<Rcvr&&>(_rcvr));
-  }
-
-  Rcvr _rcvr;
-};
-
-template <class Domain>
-struct _sndr_t
-{
-  using sender_concept = ex::sender_t;
-
-  template <class Self>
-  _CCCL_HOST_DEVICE static _CCCL_CONSTEVAL auto get_completion_signatures() noexcept
-  {
-    return ex::completion_signatures<ex::set_value_t()>();
-  }
-
-  template <class Rcvr>
-  _CCCL_HOST_DEVICE constexpr auto connect(Rcvr rcvr) const noexcept -> _opstate_t<Rcvr>
-  {
-    return _opstate_t<Rcvr>(static_cast<Rcvr&&>(rcvr));
-  }
-
-  [[nodiscard]] _CCCL_HOST_DEVICE constexpr auto get_env() const noexcept
-  {
-    return _attrs_t<Domain>{};
-  }
-};
-} // namespace _dummy
-
-//! Scheduler that returns a sender that always completes inline (successfully).
-template <class Domain = ex::default_domain>
-struct dummy_scheduler : _dummy::_attrs_t<Domain>
-{
-  using scheduler_concept = ex::scheduler_t;
-
-  _CCCL_HOST_DEVICE static constexpr auto schedule() noexcept -> _dummy::_sndr_t<Domain>
-  {
-    return {};
-  }
-
-  _CCCL_HOST_DEVICE friend constexpr bool operator==(dummy_scheduler, dummy_scheduler) noexcept
-  {
-    return true;
-  }
-
-  _CCCL_HOST_DEVICE friend constexpr bool operator!=(dummy_scheduler, dummy_scheduler) noexcept
-  {
-    return false;
-  }
-};
-
-namespace _dummy
-{
-template <class Domain>
-_CCCL_HOST_DEVICE constexpr auto _attrs_t<Domain>::query(ex::get_completion_scheduler_t<ex::set_value_t>) const noexcept
-{
-  return dummy_scheduler<Domain>{};
-}
-} // namespace _dummy
-} // namespace