[CCCL] Add missing CCCL components: c2h, nvbench_helper, cmake, cudax, AGENTS.md

Added 863 files from NVIDIA/cccl sparse checkout:
- c2h/ (27 files): Catch2 test helpers — generators, validators, runner
- nvbench_helper/ (10 files): Benchmark harness utilities
- cmake/ (29 files): CMake presets and build helpers
- cudax/ (794 files): Experimental CUDA extensions
- AGENTS.md: NVIDIA's official AI agent instructions for CCCL
- CMakePresets.json: Standardized build configurations
- cccl-version.json: Version tracking

Also added CCCL_ASSET_MAP.md mapping all 4295 CCCL files to
competition value and PRD items.

cccl_upstream now covers 100% of competition-critical assets:
- 27 tuning headers (SM80/90/100 benchmark data)
- 32 dispatch headers (algorithm implementations)
- 60 Thrust examples (correctness verification)
- 217 CUB Catch2 tests (regression matrix)
- 153 CUB benchmarks (parameter space search)
- 18 CUB examples (API verification)
- 27 test helpers + benchmark harness
- 794 cudax experimental extensions

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