[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/examples/01-axpy-launch-ranges-cg.cu

@@ -1,75 +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/__places/partitions/blocked_partition.cuh>
-#include <cuda/experimental/__places/partitions/cyclic_shape.cuh>
-#include <cuda/experimental/stf.cuh>
-
-using namespace cuda::experimental::stf;
-
-double X0(int i)
-{
-  return sin((double) i);
-}
-
-double Y0(int i)
-{
-  return cos((double) i);
-}
-
-int main()
-{
-  stream_ctx ctx;
-
-  const int N = 128;
-  double X[N], Y[N];
-
-  for (int ind = 0; ind < N; ind++)
-  {
-    X[ind] = X0(ind);
-    Y[ind] = Y0(ind);
-  }
-
-  const double alpha = 3.14;
-
-  auto handle_X = ctx.logical_data(X, {N});
-  auto handle_Y = ctx.logical_data(Y, {N});
-
-  auto number_devices = 4;
-  auto all_devs       = exec_place::repeat(exec_place::device(0), number_devices);
-
-  auto spec = par(16 * 4, par(4));
-  ctx.launch(spec, all_devs, handle_X.read(), handle_Y.rw())->*[=] _CCCL_DEVICE(auto th, auto x, auto y) {
-    // Blocked partition among elements in the outer most level
-    auto outer_sh = blocked_partition::apply(shape(x), pos4(th.rank(0)), dim4(th.size(0)));
-
-    // Cyclic partition among elements in the remaining levels
-    auto inner_sh = cyclic_partition::apply(outer_sh, pos4(th.inner().rank()), dim4(th.inner().size()));
-
-    for (auto ind : inner_sh)
-    {
-      y(ind) += alpha * x(ind);
-    }
-  };
-
-  ctx.host_launch(handle_X.read(), handle_Y.read())->*[=](auto X, auto Y) {
-    for (int ind = 0; ind < N; ind++)
-    {
-      // Y should be Y0 + alpha X0
-      // fprintf(stderr, "Y[%ld] = %lf - expect %lf\n", ind, Y(ind), (Y0(ind) + alpha * X0(ind)));
-      EXPECT(fabs(Y(ind) - (Y0(ind) + alpha * X0(ind))) < 0.0001);
-
-      // X should be X0
-      EXPECT(fabs(X(ind) - X0(ind)) < 0.0001);
-    }
-  };
-
-  ctx.finalize();
-}