[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/execution/test_sequence.cu

@@ -1,48 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// Part of CUDA Experimental 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) 2025 NVIDIA CORPORATION & AFFILIATES.
-//
-//===----------------------------------------------------------------------===//
-
-// Include this first
-#include <cuda/experimental/execution.cuh>
-
-// Then include the test helpers
-#include <nv/target>
-
-#include "common/checked_receiver.cuh"
-#include "common/utility.cuh"
-#include "testing.cuh"
-
-namespace ex = cuda::experimental::execution;
-
-namespace
-{
-C2H_TEST("simple use of sequence executes both child operations", "[adaptors][sequence]")
-{
-  bool flag1{false};
-  bool flag2{false};
-
-  auto sndr1 = ex::sequence(
-    ex::just() | ex::then([&] {
-      flag1 = true;
-    }),
-    ex::just() | ex::then([&] {
-      flag2 = true;
-    }));
-
-  check_value_types<types<>>(sndr1);
-  check_sends_stopped<false>(sndr1);
-  check_error_types<ex::exception_ptr>(sndr1);
-
-  auto op = ex::connect(std::move(sndr1), checked_value_receiver<>{});
-  ex::start(op);
-
-  CHECK(flag1);
-  CHECK(flag2);
-}
-} // namespace