[INFRA] Import NVIDIA/CCCL upstream as optimization reference library

CCCL (CUDA C++ Core Libraries) provides:
- CUB: device/block/warp-level GPU primitives (reduce, scan, sort, topk)
- Thrust: high-level parallel algorithms (transform_reduce, sort, scan)
- libcudacxx: CUDA C++ standard library (atomics, barriers, memory)
- cudax: experimental features (memory resources, allocators)
- Tuning policies: per-SM hardware-specific algorithm parameters

Competition optimization vectors mapped to CCCL:
- Output TPS (83% weight): warp_reduce, block_reduce, device_topk
- Input TPS (14% weight): device_scan, block_load, prefetch
- Cache TPS (3% weight): prefix caching strategy patterns
- Memory (0.9 util): pooled/cached/buddy allocators

Source: https://github.com/NVIDIA/cccl (shallow clone, HEAD only)
License: Apache-2.0

cccl_upstream/cudax/test/execution/test_write_attrs.cu

@@ -0,0 +1,31 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 "testing.cuh" // IWYU pragma: keep
+
+namespace
+{
+struct my_domain
+{};
+
+C2H_TEST("basic test of write_attrs", "[write_attrs]")
+{
+  auto sndr = cudax_async::just(42) | cudax_async::write_attrs(cudax_async::prop{cudax_async::get_domain, my_domain{}});
+  [[maybe_unused]] auto domain = cudax_async::get_domain(cudax_async::get_env(sndr));
+  STATIC_REQUIRE(std::is_same_v<decltype(domain), my_domain>);
+
+  // Check that the sender can be connected and started
+  auto op = cudax_async::connect(std::move(sndr), checked_value_receiver{42});
+  cudax_async::start(op);
+}
+} // namespace