[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/04-fibonacci.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.
-//
-//===----------------------------------------------------------------------===//
-
-/**
- * @file
- *
- * @brief An example of Fibonacci sequence illustrating how we can use
- *        dynamically created logical data
- */
-
-#include <cuda/experimental/stf.cuh>
-
-using namespace cuda::experimental::stf;
-
-int fibo_ref(int n)
-{
-  if (n < 2)
-  {
-    return n;
-  }
-  else
-  {
-    return fibo_ref(n - 1) + fibo_ref(n - 2);
-  }
-}
-
-__global__ void add(slice<int> out, const slice<const int> in1, const slice<const int> in2)
-{
-  out(0) = in1(0) + in2(0);
-}
-
-__global__ void set(slice<int> out, int val)
-{
-  out(0) = val;
-}
-
-logical_data<slice<int>> compute_fibo(context& ctx, int n)
-{
-  auto out = ctx.logical_data(shape_of<slice<int>>(1));
-  if (n < 2)
-  {
-    ctx.task(out.write())->*[=](cudaStream_t s, auto sout) {
-      set<<<1, 1, 0, s>>>(sout, n);
-    };
-  }
-  else
-  {
-    auto fib1 = compute_fibo(ctx, n - 1);
-    auto fib2 = compute_fibo(ctx, n - 2);
-    ctx.task(fib1.read(), fib2.read(), out.write())->*[=](cudaStream_t s, auto s1, auto s2, auto sout) {
-      add<<<1, 1, 0, s>>>(sout, s1, s2);
-    };
-  }
-
-  return out;
-}
-
-int main(int argc, char** argv)
-{
-  int n = (argc > 1) ? atoi(argv[1]) : 4;
-
-  context ctx; // = graph_ctx();
-  auto result = compute_fibo(ctx, n);
-  ctx.host_launch(result.read())->*[&](auto res) {
-    EXPECT(res(0) == fibo_ref(n));
-  };
-  ctx.finalize();
-}