[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_task_scheduler.cu

@@ -1,94 +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/checked_receiver.cuh" // IWYU pragma: keep
-#include "common/dummy_scheduler.cuh" // IWYU pragma: keep
-#include "common/error_scheduler.cuh" // IWYU pragma: keep
-#include "common/stopped_scheduler.cuh" // IWYU pragma: keep
-#include "common/utility.cuh" // IWYU pragma: keep
-
-namespace ex = cuda::experimental::execution;
-
-namespace
-{
-C2H_TEST("simple task_scheduler test", "[scheduler][task_scheduler]")
-{
-  ex::task_scheduler sched{dummy_scheduler{}};
-  STATIC_CHECK(ex::scheduler<decltype(sched)>);
-  auto sndr = sched.schedule();
-  STATIC_CHECK(ex::sender<decltype(sndr)>);
-  auto op = ex::connect(cuda::std::move(sndr), checked_value_receiver{});
-  ex::start(op);
-  // The receiver checks that it's called
-}
-
-C2H_TEST("task_scheduler starts work on the correct execution context", "[scheduler][task_scheduler]")
-{
-  ex::thread_context ctx;
-  ex::task_scheduler sched{ctx.get_scheduler()};
-  auto sndr  = ex::starts_on(sched, ex::just() | ex::then([] {
-                                     return ::std::this_thread::get_id();
-                                   }));
-  auto [tid] = ex::sync_wait(cuda::std::move(sndr)).value();
-  CHECK(tid == ctx.get_id());
-}
-
-#if !_CCCL_HOST_COMPILATION()
-static __device__ bool g_called = false;
-#else
-static bool g_called = false;
-#endif
-
-template <class Sndr>
-struct protect : private Sndr
-{
-  using sender_concept = ex::sender_t;
-  _CCCL_HOST_DEVICE_API explicit protect(Sndr sndr)
-      : Sndr{cuda::std::move(sndr)}
-  {}
-  using Sndr::connect;
-  using Sndr::get_completion_signatures;
-  using Sndr::get_env;
-};
-
-struct test_domain
-{
-  _CCCL_TEMPLATE(class Sndr, class Env)
-  _CCCL_REQUIRES(ex::sender_for<Sndr, ex::bulk_chunked_t>)
-  _CCCL_HOST_DEVICE_API auto transform_sender(ex::set_value_t, Sndr sndr, const Env&) const
-  {
-    return ex::then(protect{cuda::std::move(sndr)}, []() noexcept {
-      g_called = true;
-    });
-  }
-};
-
-C2H_TEST("bulk_unchunked dispatches correctly through task_scheduler", "[scheduler][task_scheduler]")
-{
-  ex::task_scheduler sched{dummy_scheduler<test_domain>{}};
-  auto sndr  = ex::on(sched, ex::just(-1) | ex::bulk_chunked(ex::par_unseq, 100, [](int, int, int&) {}));
-  g_called   = false;
-  auto [val] = ex::sync_wait(cuda::std::move(sndr)).value();
-  CHECK(val == -1);
-  CHECK(g_called);
-}
-
-C2H_TEST("bulk dispatches correctly through task_scheduler", "[scheduler][task_scheduler]")
-{
-  ex::task_scheduler sched{dummy_scheduler<test_domain>{}};
-  auto sndr  = ex::on(sched, ex::just(-1) | ex::bulk(ex::par_unseq, 100, [](int, int&) {}));
-  g_called   = false;
-  auto [val] = ex::sync_wait(cuda::std::move(sndr)).value();
-  CHECK(val == -1);
-  CHECK(g_called);
-}
-} // namespace