[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/test_trampoline_scheduler.cu

@@ -1,97 +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 <cuda/experimental/execution.cuh>
-
-#include "./common/retry.cuh"
-#include "testing.cuh"
-
-namespace ex = ::cuda::experimental::execution;
-
-#if !_CCCL_DEVICE_COMPILATION()
-
-namespace
-{
-struct try_again
-{};
-
-class fails_alot
-{
-  template <class Receiver>
-  struct operation;
-
-public:
-  using sender_concept = ex::sender_t;
-
-  fails_alot() = default;
-
-  __host__ fails_alot(fails_alot&& other) noexcept
-      : counter_(std::move(other.counter_))
-  {}
-
-  __host__ fails_alot(const fails_alot& other) noexcept
-      : counter_(other.counter_)
-  {}
-
-  template <class Receiver>
-  [[nodiscard]] auto connect(Receiver rcvr) const noexcept -> operation<Receiver>
-  {
-    return operation<Receiver>{static_cast<Receiver&&>(rcvr), --*counter_};
-  }
-
-  template <class...>
-  static _CCCL_CONSTEVAL auto get_completion_signatures() noexcept
-  {
-    return ex::completion_signatures<ex::set_value_t(), ex::set_error_t(try_again)>{};
-  }
-
-private:
-  template <class Receiver>
-  struct operation
-  {
-    void start() & noexcept
-    {
-      if (counter_ == 0)
-      {
-        ex::set_value(static_cast<Receiver&&>(rcvr_));
-      }
-      else
-      {
-        ex::set_error(static_cast<Receiver&&>(rcvr_), try_again{});
-      }
-    }
-
-    Receiver rcvr_;
-    int counter_;
-  };
-
-  std::shared_ptr<int> counter_ = std::make_shared<int>(1'000'000);
-};
-
-// #if defined(REQUIRE_TERMINATE)
-// // For some reason, when compiling with nvc++, the forked process dies with SIGSEGV
-// // but the error code returned from ::wait reports success, so this test fails.
-// TEST_CASE("running deeply recursing algo blows the stack", "[schedulers][trampoline_scheduler]") {
-
-//   auto recurse_deeply = retry(fails_alot{});
-//   REQUIRE_TERMINATE([&] { sync_wait(std::move(recurse_deeply)); });
-// }
-// #endif
-
-TEST_CASE("running deeply recursing algo on trampoline_scheduler doesn't blow the stack",
-          "[scheduler][trampoline_scheduler]")
-{
-  ex::trampoline_scheduler sched;
-  auto recurse_deeply = retry(ex::on(sched, fails_alot{}));
-  ex::sync_wait(std::move(recurse_deeply));
-}
-} // namespace
-
-#endif // !_CCCL_DEVICE_COMPILATION()