[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_write_env.cu

@@ -1,98 +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 <cuda/experimental/execution.cuh>
-
-#include "common/checked_receiver.cuh"
-#include "common/utility.cuh"
-#include "testing.cuh"
-
-namespace ex = cuda::experimental::execution;
-
-namespace
-{
-// Custom domain for testing
-struct my_domain
-{};
-
-// Simple test query
-struct query1_t : ex::forwarding_query_t
-{
-  template <class Env>
-  _CCCL_HOST_DEVICE_API auto operator()(const Env& env) const noexcept -> decltype(env.query(*this))
-  {
-    return env.query(*this);
-  }
-};
-
-inline constexpr auto query1 = query1_t{};
-} // namespace
-
-C2H_TEST("write_env basic functionality", "[write_env][adaptors]")
-{
-  // Test that write_env wraps a sender and adds environment information
-  auto env  = ex::prop{query1, 42};
-  auto sndr = ex::write_env(ex::just(42), env);
-
-  auto op = ex::connect(std::move(sndr), checked_value_receiver{42});
-  ex::start(op);
-  // the receiver will check the result
-}
-
-C2H_TEST("write_env pipe syntax", "[write_env][adaptors]")
-{
-  // Test that write_env supports pipe syntax (sndr | write_env(env))
-  auto env  = ex::prop{query1, 42};
-  auto sndr = ex::just(42) | ex::write_env(env);
-
-  auto op = ex::connect(std::move(sndr), checked_value_receiver{42});
-  ex::start(op);
-  // the receiver will check the result
-}
-
-#if _CCCL_HOST_COMPILATION()
-C2H_TEST("write_env updates the receiver's environment", "[write_env][adaptors]")
-{
-  auto sndr = ex::just() | ex::let_value([]() {
-                return ex::read_env(query1);
-              })
-            | ex::write_env(ex::prop{query1, 42});
-
-  auto [result] = ex::sync_wait(std::move(sndr)).value();
-  CHECK(result == 42);
-}
-
-struct fake_allocator
-{};
-
-C2H_TEST("write_env does not hide the receiver's environment", "[write_env][adaptors]")
-{
-  auto sndr = ex::just() | ex::let_value([]() {
-                return ex::read_env(ex::get_allocator);
-              })
-            | ex::write_env(ex::prop{query1, 42});
-
-  auto env      = ex::prop{ex::get_allocator, fake_allocator{}};
-  auto [result] = ex::sync_wait(std::move(sndr), env).value();
-  STATIC_REQUIRE(std::is_same_v<decltype(result), fake_allocator>);
-}
-
-C2H_TEST("write_env prefers the passed env over the receiver's env", "[write_env][adaptors]")
-{
-  auto sndr = ex::just() | ex::let_value([]() {
-                return ex::read_env(query1);
-              })
-            | ex::write_env(ex::prop{query1, 42});
-
-  auto env      = ex::prop{query1, 100};
-  auto [result] = ex::sync_wait(std::move(sndr), env).value();
-  CHECK(result == 42);
-}
-#endif // _CCCL_HOST_COMPILATION()