[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/common/dummy_scheduler.cuh

@@ -0,0 +1,107 @@
+//===----------------------------------------------------------------------===//
+//
+// 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.
+//
+//===----------------------------------------------------------------------===//
+
+#pragma once
+
+#include <cuda/__utility/immovable.h>
+
+#include <cuda/experimental/execution.cuh>
+
+#include "testing.cuh" // IWYU pragma: keep
+
+namespace ex = cuda::experimental::execution;
+
+namespace
+{
+namespace _dummy
+{
+template <class Domain>
+struct _attrs_t
+{
+  _CCCL_HOST_DEVICE constexpr auto query(ex::get_completion_scheduler_t<ex::set_value_t>) const noexcept;
+
+  _CCCL_HOST_DEVICE constexpr auto query(ex::get_completion_domain_t<ex::set_value_t>) const noexcept
+  {
+    return Domain{};
+  }
+};
+
+template <class Rcvr>
+struct _opstate_t : cuda::__immovable
+{
+  using operation_state_concept = ex::operation_state_t;
+
+  _CCCL_HOST_DEVICE constexpr _opstate_t(Rcvr rcvr) noexcept
+      : _rcvr(static_cast<Rcvr&&>(rcvr))
+  {}
+
+  _CCCL_HOST_DEVICE constexpr void start() noexcept
+  {
+    ex::set_value(static_cast<Rcvr&&>(_rcvr));
+  }
+
+  Rcvr _rcvr;
+};
+
+template <class Domain>
+struct _sndr_t
+{
+  using sender_concept = ex::sender_t;
+
+  template <class Self>
+  _CCCL_HOST_DEVICE static _CCCL_CONSTEVAL auto get_completion_signatures() noexcept
+  {
+    return ex::completion_signatures<ex::set_value_t()>();
+  }
+
+  template <class Rcvr>
+  _CCCL_HOST_DEVICE constexpr auto connect(Rcvr rcvr) const noexcept -> _opstate_t<Rcvr>
+  {
+    return _opstate_t<Rcvr>(static_cast<Rcvr&&>(rcvr));
+  }
+
+  [[nodiscard]] _CCCL_HOST_DEVICE constexpr auto get_env() const noexcept
+  {
+    return _attrs_t<Domain>{};
+  }
+};
+} // namespace _dummy
+
+//! Scheduler that returns a sender that always completes inline (successfully).
+template <class Domain = ex::default_domain>
+struct dummy_scheduler : _dummy::_attrs_t<Domain>
+{
+  using scheduler_concept = ex::scheduler_t;
+
+  _CCCL_HOST_DEVICE static constexpr auto schedule() noexcept -> _dummy::_sndr_t<Domain>
+  {
+    return {};
+  }
+
+  _CCCL_HOST_DEVICE friend constexpr bool operator==(dummy_scheduler, dummy_scheduler) noexcept
+  {
+    return true;
+  }
+
+  _CCCL_HOST_DEVICE friend constexpr bool operator!=(dummy_scheduler, dummy_scheduler) noexcept
+  {
+    return false;
+  }
+};
+
+namespace _dummy
+{
+template <class Domain>
+_CCCL_HOST_DEVICE constexpr auto _attrs_t<Domain>::query(ex::get_completion_scheduler_t<ex::set_value_t>) const noexcept
+{
+  return dummy_scheduler<Domain>{};
+}
+} // namespace _dummy
+} // namespace