[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/examples/stf/word_count.cu

@@ -1,91 +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 Counting words in a text using a launch kernel
- */
-
-#include <cuda/experimental/stf.cuh>
-
-using namespace cuda::experimental::stf;
-
-// determines whether the character is alphabetical
-__host__ __device__ bool is_alpha(const char c)
-{
-  return (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z');
-}
-
-int main()
-{
-  // Paragraph from 'The Raven' by Edgar Allan Poe
-  // http://en.wikipedia.org/wiki/The_Raven
-  const char raw_input[] =
-    "  But the raven, sitting lonely on the placid bust, spoke only,\n"
-    "  That one word, as if his soul in that one word he did outpour.\n"
-    "  Nothing further then he uttered - not a feather then he fluttered -\n"
-    "  Till I scarcely more than muttered `Other friends have flown before -\n"
-    "  On the morrow he will leave me, as my hopes have flown before.'\n"
-    "  Then the bird said, `Nevermore.'\n";
-
-  context ctx;
-
-  auto ltext = ctx.logical_data(const_cast<char*>(&raw_input[0]), {sizeof(raw_input)});
-
-  int cnt   = 0;
-  auto lcnt = ctx.logical_data(&cnt, {1});
-
-  auto number_devices = 2;
-  auto all_devs       = exec_place::repeat(exec_place::device(0), number_devices);
-
-  auto spec = par(con(128));
-
-  ctx.launch(spec, all_devs, ltext.read(), lcnt.rw())->*[] _CCCL_DEVICE(auto th, auto text, auto cnt) {
-    int local_cnt = 0;
-    for (size_t i = th.rank(); i < text.size() - 1; i += th.size())
-    {
-      /* If the thread encounters the beginning of a new word, increment
-       * its local counter */
-      if (!is_alpha(text(i)) && is_alpha(text(i + 1)))
-      {
-        local_cnt++;
-      }
-    }
-
-    // Get a piece of shared memory, and zero it
-    __shared__ int block_cnt;
-    block_cnt = 0;
-    th.inner().sync();
-
-    // In every block, partial sums are gathered, and added to the result
-    // by the first thread of the block.
-    atomicAdd(&block_cnt, local_cnt);
-    th.inner().sync();
-
-    if (th.inner().rank() == 0)
-    {
-      atomicAdd(&cnt(0), block_cnt);
-    }
-  };
-
-  ctx.finalize();
-
-  int ref_cnt = 0;
-  for (size_t i = 0; i < sizeof(raw_input) - 1; i++)
-  {
-    if (!is_alpha(raw_input[i]) && is_alpha(raw_input[i + 1]))
-    {
-      ref_cnt++;
-    }
-  }
-
-  // fprintf(stderr, "Result : found %d words (expected %d)\n", cnt, ref_cnt);
-  EXPECT(cnt == ref_cnt);
-}