[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/linear_algebra/dot.cuh

@@ -1,70 +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.
-//
-//===----------------------------------------------------------------------===//
-
-#pragma once
-
-//! \file
-//! \brief DOT algorithm
-
-#include <cuda/experimental/stf.cuh>
-
-using namespace cuda::experimental::stf;
-
-template <typename T>
-using vector_t = stackable_logical_data<slice<T>>;
-
-template <typename T>
-using scalar_t = stackable_logical_data<scalar_view<T>>;
-
-template <typename T = double>
-struct csr_matrix
-{
-  csr_matrix(stackable_logical_data<slice<T>> _val_handle,
-             stackable_logical_data<slice<size_t>> _row_handle,
-             stackable_logical_data<slice<size_t>> _col_handle)
-      : val_handle(mv(_val_handle))
-      , row_handle(mv(_row_handle))
-      , col_handle(mv(_col_handle))
-  {}
-
-  /* Description of the CSR */
-  mutable stackable_logical_data<slice<T>> val_handle;
-  mutable stackable_logical_data<slice<size_t>> row_handle;
-  mutable stackable_logical_data<slice<size_t>> col_handle;
-};
-
-// Note that a and b might be the same logical data
-template <typename ctx_t, typename T>
-void DOT(ctx_t& ctx, vector_t<T>& a, vector_t<T>& b, scalar_t<T>& res)
-{
-  ctx.parallel_for(a.shape(), a.read(), b.read(), res.reduce(reducer::sum<T>{})).set_symbol("DOT")->*
-    [] __device__(size_t i, auto da, auto db, T& dres) {
-      dres += da(i) * db(i);
-    };
-};
-
-template <typename ctx_t, typename T>
-void SPMV(ctx_t& ctx, csr_matrix<T>& a, vector_t<T>& x, vector_t<T>& y)
-{
-  ctx.parallel_for(y.shape(), a.val_handle.read(), a.col_handle.read(), a.row_handle.read(), x.read(), y.write())
-      .set_symbol("SPMV")
-      ->*[] _CCCL_DEVICE(size_t row, auto da_val, auto da_col, auto da_row, auto dx, auto dy) {
-            int row_start = da_row(row);
-            int row_end   = da_row(row + 1);
-
-            double sum = 0.0;
-            for (int elt = row_start; elt < row_end; elt++)
-            {
-              sum += da_val(elt) * dx(da_col(elt));
-            }
-
-            dy(row) = sum;
-          };
-}