[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/stopped_scheduler.cuh

@@ -0,0 +1,120 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+{
+struct _stopped_scheduler_attrs_t
+{
+  template <class _Env>
+  _CCCL_HOST_DEVICE auto
+  query(cudax_async::get_completion_scheduler_t<cudax_async::set_value_t>, const _Env& env) const noexcept
+    -> decltype(cudax_async::get_completion_scheduler<cudax_async::set_value_t>(env, env))
+  {
+    return cudax_async::get_completion_scheduler<cudax_async::set_value_t>(env, env);
+  }
+
+  template <class _Env>
+  _CCCL_HOST_DEVICE auto
+  query(cudax_async::get_completion_scheduler_t<cudax_async::set_stopped_t>, const _Env& env) const noexcept
+    -> decltype(cudax_async::get_completion_scheduler<cudax_async::set_stopped_t>(env, env))
+  {
+    return cudax_async::get_completion_scheduler<cudax_async::set_stopped_t>(env, env);
+  }
+
+  template <class _Env>
+  _CCCL_HOST_DEVICE auto
+  query(cudax_async::get_completion_domain_t<cudax_async::set_value_t>, const _Env& env) const noexcept
+    -> decltype(cudax_async::get_completion_domain<cudax_async::set_value_t>(env, env))
+  {
+    return cudax_async::get_completion_domain<cudax_async::set_value_t>(env, env);
+  }
+
+  template <class _Env>
+  _CCCL_HOST_DEVICE auto
+  query(cudax_async::get_completion_domain_t<cudax_async::set_stopped_t>, const _Env& env) const noexcept
+    -> decltype(cudax_async::get_completion_domain<cudax_async::set_stopped_t>(env, env))
+  {
+    return cudax_async::get_completion_domain<cudax_async::set_stopped_t>(env, env);
+  }
+
+  _CCCL_HOST_DEVICE static constexpr auto query(cudax_async::get_completion_behavior_t) noexcept
+  {
+    return cudax_async::completion_behavior::inline_completion;
+  }
+};
+
+//! Scheduler that returns a sender that always completes with stopped.
+struct stopped_scheduler : _stopped_scheduler_attrs_t
+{
+private:
+  template <class Rcvr>
+  struct _opstate_t : cuda::__immovable
+  {
+    using operation_state_concept = cudax_async::operation_state_t;
+
+    Rcvr _rcvr;
+
+    _CCCL_HOST_DEVICE void start() noexcept
+    {
+      cudax_async::set_stopped(static_cast<Rcvr&&>(_rcvr));
+    }
+  };
+
+  struct _sndr_t
+  {
+    using sender_concept = cudax_async::sender_t;
+
+    template <class Self>
+    _CCCL_HOST_DEVICE static constexpr auto get_completion_signatures()
+    {
+      return cudax_async::completion_signatures<cudax_async::set_value_t(), cudax_async::set_stopped_t()>();
+    }
+
+    template <class Rcvr>
+    _CCCL_HOST_DEVICE auto connect(Rcvr rcvr) const noexcept -> _opstate_t<Rcvr>
+    {
+      return {{}, static_cast<Rcvr&&>(rcvr)};
+    }
+
+    _CCCL_HOST_DEVICE auto get_env() const noexcept -> _stopped_scheduler_attrs_t
+    {
+      return {};
+    }
+  };
+
+public:
+  using scheduler_concept = cudax_async::scheduler_t;
+
+  stopped_scheduler() = default;
+
+  _CCCL_HOST_DEVICE auto schedule() const noexcept -> _sndr_t
+  {
+    return {};
+  }
+
+  _CCCL_HOST_DEVICE friend constexpr bool operator==(stopped_scheduler, stopped_scheduler) noexcept
+  {
+    return true;
+  }
+
+  _CCCL_HOST_DEVICE friend constexpr bool operator!=(stopped_scheduler, stopped_scheduler) noexcept
+  {
+    return false;
+  }
+};
+} // namespace