[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/group/make_this_group.cu

@@ -0,0 +1,79 @@
+//===----------------------------------------------------------------------===//
+//
+// 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.
+//
+//===----------------------------------------------------------------------===//
+
+#include <cuda/devices>
+#include <cuda/hierarchy>
+#include <cuda/launch>
+#include <cuda/std/type_traits>
+#include <cuda/stream>
+
+#include <cuda/experimental/group.cuh>
+
+#include "group_testing.cuh"
+
+namespace
+{
+template <template <class> class GroupTempl, class Level, class Config>
+__device__ void test_make_this_group(const Level& level, const Config& config)
+{
+  // Test default construction.
+  {
+    static_assert(
+      cuda::std::is_same_v<GroupTempl<cudax::__implicit_hierarchy_t>, decltype(cudax::make_this_group(level))>);
+    static_assert(noexcept(cudax::make_this_group(level)));
+
+    auto group = cudax::make_this_group(level);
+    group.sync();
+  }
+
+  // Test construction from hierarchy-like.
+  {
+    using Hierarchy = typename Config::hierarchy_type;
+
+    static_assert(cuda::std::is_same_v<GroupTempl<Hierarchy>, decltype(cudax::make_this_group(level, config))>);
+    static_assert(noexcept(cudax::make_this_group(level, config)));
+
+    auto group = cudax::make_this_group(level, config);
+    group.sync();
+  }
+}
+
+struct TestKernel
+{
+  template <class Config>
+  __device__ void operator()(const Config& config)
+  {
+    test_make_this_group<cudax::this_thread>(cuda::gpu_thread, config);
+    test_make_this_group<cudax::this_warp>(cuda::warp, config);
+    test_make_this_group<cudax::this_block>(cuda::block, config);
+    test_make_this_group<cudax::this_cluster>(cuda::cluster, config);
+    test_make_this_group<cudax::this_grid>(cuda::grid, config);
+  }
+};
+} // namespace
+
+C2H_TEST("Make this group", "[group]")
+{
+  const auto device = cuda::devices[0];
+
+  const cuda::stream stream{device};
+
+  const auto config = cuda::make_config(cuda::grid_dims<2>(), cuda::block_dims<128>(), cuda::cooperative_launch{});
+  cuda::launch(stream, config, TestKernel{});
+
+  if (cuda::device_attributes::compute_capability(device) >= cuda::compute_capability{90})
+  {
+    const auto config_cluster = cuda::make_config(
+      cuda::grid_dims<2>(), cuda::cluster_dims<3>(), cuda::block_dims<128>(), cuda::cooperative_launch{});
+    cuda::launch(stream, config_cluster, TestKernel{});
+  }
+
+  stream.sync();
+}