[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/include/cuda/experimental/graph.cuh

@@ -0,0 +1,30 @@
+//===----------------------------------------------------------------------===//
+//
+// 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) 2026 NVIDIA CORPORATION & AFFILIATES.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef __CUDAX_GRAPH_CUH
+#define __CUDAX_GRAPH_CUH
+
+// IWYU pragma: begin_exports
+#include <cuda/experimental/__graph/child_graph.cuh>
+#include <cuda/experimental/__graph/conditional_node.cuh>
+#include <cuda/experimental/__graph/copy_bytes.cuh>
+#include <cuda/experimental/__graph/depends_on.cuh>
+#include <cuda/experimental/__graph/fill_bytes.cuh>
+#include <cuda/experimental/__graph/fwd.cuh>
+#include <cuda/experimental/__graph/graph.cuh>
+#include <cuda/experimental/__graph/graph_builder.cuh>
+#include <cuda/experimental/__graph/graph_memory_resource.cuh>
+#include <cuda/experimental/__graph/graph_node_ref.cuh>
+#include <cuda/experimental/__graph/graph_node_type.cuh>
+#include <cuda/experimental/__graph/host_launch.cuh>
+#include <cuda/experimental/__graph/path_builder.cuh>
+// IWYU pragma: end_exports
+
+#endif // __CUDAX_GRAPH_CUH