[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/hashtable/fusion.cu

@@ -1,64 +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.
-//
-//===----------------------------------------------------------------------===//
-
-#include <cuda/experimental/__stf/stream/interfaces/hashtable_linearprobing.cuh>
-#include <cuda/experimental/__stf/stream/stream_ctx.cuh>
-
-using namespace cuda::experimental::stf;
-
-// Iterate over every item in the hashtableA, and add them to B
-
-__global__ void gpu_merge_hashtable(hashtable A, const hashtable B)
-{
-  unsigned int threadid = blockIdx.x * blockDim.x + threadIdx.x;
-  while (threadid < reserved::kHashTableCapacity)
-  {
-    if (B.addr[threadid].key != reserved::kEmpty)
-    {
-      uint32_t value = B.addr[threadid].value;
-      if (value != reserved::kEmpty)
-      {
-        //    printf("INSERTING key %d value %d\n", pHashTableB[threadid].key, value);
-        A.insert(B.addr[threadid]);
-      }
-    }
-    threadid += blockDim.x * gridDim.x;
-  }
-}
-
-int main()
-{
-  stream_ctx ctx;
-
-  hashtable A;
-  A.insert(reserved::KeyValue(107, 4));
-  A.insert(reserved::KeyValue(108, 6));
-
-  hashtable B;
-  B.insert(reserved::KeyValue(7, 14));
-  B.insert(reserved::KeyValue(8, 16));
-
-  auto lA = ctx.logical_data(A);
-  auto lB = ctx.logical_data(B);
-
-  ctx.task(lA.rw(), lB.read())->*[](auto stream, auto hA, auto hB) {
-    gpu_merge_hashtable<<<32, 128, 0, stream>>>(hA, hB);
-    cuda_safe_call(cudaGetLastError());
-  };
-
-  ctx.host_launch(lA.read())->*[](auto hA) {
-    EXPECT(hA.get(107) == 4);
-    EXPECT(hA.get(108) == 6);
-    EXPECT(hA.get(7) == 14);
-    EXPECT(hA.get(8) == 16);
-  };
-
-  ctx.finalize();
-}