[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_reduction.cu

@@ -1,122 +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/reduction.cuh>
-#include <cuda/experimental/__stf/stream/stream_ctx.cuh>
-
-using namespace cuda::experimental::stf;
-
-__global__ void gpu_merge_hashtable(hashtable A, const hashtable B)
-{
-  unsigned int threadid = blockIdx.x * blockDim.x + threadIdx.x;
-  while (threadid < B.get_capacity())
-  {
-    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;
-  }
-}
-
-void cpu_merge_hashtable(hashtable A, const hashtable B)
-{
-  for (unsigned int i = 0; i < B.get_capacity(); i++)
-  {
-    if (B.addr[i].key != reserved::kEmpty)
-    {
-      uint32_t value = B.addr[i].value;
-      if (value != reserved::kEmpty)
-      {
-        //    printf("INSERTING key %d value %d\n", pHashTableB[threadid].key, value);
-        A.insert(B.addr[i]);
-      }
-    }
-  }
-}
-
-class hashtable_fusion_t : public stream_reduction_operator<hashtable>
-{
-  void op(const hashtable& in, hashtable& inout, const exec_place& e, cudaStream_t s) override
-  {
-    if (e.affine_data_place().is_host())
-    {
-      cuda_safe_call(cudaStreamSynchronize(s)); // TODO use a callback
-      cpu_merge_hashtable(inout, in);
-    }
-    else
-    {
-      gpu_merge_hashtable<<<32, 32, 0, s>>>(inout, in);
-    }
-  }
-
-  void init_op(hashtable& /*unused*/, const exec_place& /*unused*/, cudaStream_t /*unused*/) override
-  {
-    // This init operator is a no-op because hashtables are already
-    // initialized as empty tables
-  }
-};
-
-// A kernel to fill the hashtable with some fictitious values
-__global__ void fill_table(size_t dev_id, size_t cnt, hashtable h)
-{
-  unsigned int threadid = blockIdx.x * blockDim.x + threadIdx.x;
-  unsigned int nthreads = blockDim.x * gridDim.x;
-
-  for (unsigned int i = threadid; i < cnt; i += nthreads)
-  {
-    uint32_t key   = dev_id * 1000 + i;
-    uint32_t value = 2 * i;
-
-    reserved::KeyValue kvs(key, value);
-    h.insert(kvs);
-  }
-}
-
-int main()
-{
-  stream_ctx ctx;
-
-  // Explicit capacity of 2048 entries
-  hashtable refh(2048);
-  auto h_handle = ctx.logical_data(refh);
-
-  auto fusion_op = std::make_shared<hashtable_fusion_t>();
-
-  for (size_t dev_id = 0; dev_id < 4; dev_id++)
-  {
-    ctx.task(h_handle.relaxed(fusion_op))->*[&](auto stream, auto h) {
-      EXPECT(h.get_capacity() == 2048);
-      fill_table<<<32, 32, 0, stream>>>(dev_id, 10, h);
-    };
-  }
-
-  ctx.host_launch(h_handle.read())->*[&](auto h) {
-    // Check that the table contains all values
-    for (size_t dev_id = 0; dev_id < 4; dev_id++)
-    {
-      for (unsigned i = 0; i < 10; i++)
-      {
-        uint32_t key   = static_cast<uint32_t>(dev_id * 1000 + i);
-        uint32_t value = 2 * i;
-
-        EXPECT(h.get(key) == value);
-      }
-    }
-  };
-
-  ctx.finalize();
-}