[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/algorithm/fill.cu

@@ -0,0 +1,93 @@
+//===----------------------------------------------------------------------===//
+//
+// 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) 2024 NVIDIA CORPORATION & AFFILIATES.
+//
+//===----------------------------------------------------------------------===//
+
+#include "common.cuh"
+
+C2H_TEST("Fill", "[data_manipulation]")
+{
+  cuda::stream _stream{cuda::device_ref{0}};
+  SECTION("Host resource")
+  {
+    cuda::mr::legacy_pinned_memory_resource host_resource;
+    cudax::uninitialized_buffer<int, cuda::mr::device_accessible> buffer(host_resource, buffer_size);
+
+    cuda::fill_bytes(_stream, buffer, fill_byte);
+
+    check_result_and_erase(_stream, cuda::std::span(buffer));
+  }
+
+  SECTION("Device resource")
+  {
+    cuda::device_memory_pool_ref device_resource = cuda::device_default_memory_pool(cuda::device_ref{0});
+    cudax::uninitialized_buffer<int, cuda::mr::device_accessible> buffer(device_resource, buffer_size);
+    cuda::fill_bytes(_stream, buffer, fill_byte);
+
+    std::vector<int> host_vector(42);
+    REQUIRE_CUDART(cudaMemcpyAsync(
+      host_vector.data(), buffer.data(), buffer.size() * sizeof(int), cudaMemcpyDefault, _stream.get()));
+
+    check_result_and_erase(_stream, host_vector);
+  }
+  SECTION("Launch transform")
+  {
+    cuda::mr::legacy_pinned_memory_resource host_resource;
+    cudax::weird_buffer buffer(host_resource, buffer_size);
+
+    cuda::fill_bytes(_stream, buffer, fill_byte);
+    check_result_and_erase(_stream, cuda::std::span(buffer.data, buffer.size));
+  }
+}
+
+C2H_TEST("Mdspan Fill", "[data_manipulation]")
+{
+  cuda::stream stream{cuda::device_ref{0}};
+  {
+    cuda::std::dextents<size_t, 3> dynamic_extents{1, 2, 3};
+    auto buffer = make_buffer_for_mdspan(dynamic_extents, 0);
+    cuda::std::mdspan<int, decltype(dynamic_extents)> dynamic_mdspan(buffer.data(), dynamic_extents);
+
+    cuda::fill_bytes(stream, dynamic_mdspan, fill_byte);
+    check_result_and_erase(stream, cuda::std::span(buffer.data(), buffer.size()));
+  }
+  {
+    cuda::std::extents<size_t, 2, cuda::std::dynamic_extent, 4> mixed_extents{1};
+    auto buffer = make_buffer_for_mdspan(mixed_extents, 0);
+    cuda::std::mdspan<int, decltype(mixed_extents)> mixed_mdspan(buffer.data(), mixed_extents);
+
+    cuda::fill_bytes(stream, cuda::std::move(mixed_mdspan), fill_byte);
+    check_result_and_erase(stream, cuda::std::span(buffer.data(), buffer.size()));
+  }
+  {
+    cuda::mr::legacy_pinned_memory_resource host_resource;
+    using static_extents = cuda::std::extents<size_t, 2, 3, 4>;
+    auto size            = cuda::std::layout_left::mapping<static_extents>().required_span_size();
+    cudax::weird_buffer<cuda::std::mdspan<int, static_extents>> buffer(host_resource, size);
+
+    cuda::fill_bytes(stream, buffer, fill_byte);
+    check_result_and_erase(stream, cuda::std::span(buffer.data, buffer.size));
+  }
+}
+
+C2H_TEST("Non exhaustive mdspan fill_bytes", "[data_manipulation]")
+{
+  cuda::stream stream{cuda::device_ref{0}};
+  {
+    auto fake_strided_mdspan = create_fake_strided_mdspan();
+
+    try
+    {
+      cuda::fill_bytes(stream, fake_strided_mdspan, fill_byte);
+    }
+    catch (const ::std::invalid_argument& e)
+    {
+      CHECK(e.what() == ::std::string("fill_bytes supports only exhaustive mdspans"));
+    }
+  }
+}