[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/cpp/wait_token.cu

@@ -1,60 +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 Test ctx.wait() on a token: a blocking, value-less synchronization
-
-#include <cuda/experimental/stf.cuh>
-
-#include <type_traits>
-
-using namespace cuda::experimental::stf;
-
-__global__ void set_value(int* p, int v)
-{
-  *p = v;
-}
-
-template <typename context_t>
-void run()
-{
-  context_t ctx;
-
-  // Externally owned buffer: STF only schedules around it, it never owns it.
-  int* d_val = nullptr;
-  cuda_safe_call(cudaMalloc(&d_val, sizeof(int)));
-
-  auto tok = ctx.token();
-
-  ctx.task(tok.write())->*[=](cudaStream_t s) {
-    set_value<<<1, 1, 0, s>>>(d_val, 42);
-  };
-
-  // wait(token) has no value to materialize: it must return void and only
-  // block the host until the token's producing work has completed.
-  static_assert(::std::is_void_v<decltype(ctx.wait(tok))>, "wait(token) must return void");
-  ctx.wait(tok);
-
-  int h_val = 0;
-  cuda_safe_call(cudaMemcpy(&h_val, d_val, sizeof(int), cudaMemcpyDeviceToHost));
-  _CCCL_ASSERT(h_val == 42, "wait(token) did not synchronize the producing task");
-
-  ctx.finalize();
-  cuda_safe_call(cudaFree(d_val));
-}
-
-int main()
-{
-  run<stream_ctx>();
-  run<graph_ctx>();
-  run<context>();
-  run<stackable_ctx>();
-}