[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/test_concepts.cu

@@ -0,0 +1,86 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 <cuda/experimental/execution.cuh>
+
+#include "common/dummy_scheduler.cuh"
+#include "testing.cuh" // IWYU pragma: keep
+
+namespace ex = ::cuda::experimental::execution;
+
+struct not_a_receiver
+{};
+
+struct a_receiver
+{
+  using receiver_concept   = ex::receiver_t;
+  a_receiver(a_receiver&&) = default;
+
+  void set_value(int) && noexcept {}
+  void set_stopped() && noexcept {}
+};
+
+C2H_TEST("tests for the receiver concepts", "[concepts]")
+{
+  static_assert(!ex::receiver<not_a_receiver>);
+  static_assert(ex::receiver<a_receiver>);
+
+  using yes_completions = ex::completion_signatures<ex::set_value_t(int), ex::set_stopped_t()>;
+  static_assert(ex::receiver_of<a_receiver, yes_completions>);
+
+  using no_completions =
+    ex::completion_signatures<ex::set_value_t(int), ex::set_stopped_t(), ex::set_error_t(ex::exception_ptr)>;
+  static_assert(!ex::receiver_of<a_receiver, no_completions>);
+}
+
+struct not_a_sender
+{};
+
+struct a_sender
+{
+  using sender_concept = ex::sender_t;
+
+  template <class _Self>
+  static constexpr auto get_completion_signatures()
+  {
+    return ex::completion_signatures<ex::set_value_t(int), ex::set_stopped_t()>{};
+  }
+};
+
+struct non_constexpr_complsigs
+{
+  using sender_concept = ex::sender_t;
+
+  template <class _Self, class...>
+  _CCCL_HOST_DEVICE static auto get_completion_signatures()
+  {
+    return ex::completion_signatures<ex::set_value_t(int), ex::set_stopped_t()>{};
+  }
+};
+
+C2H_TEST("tests for the sender concepts", "[concepts]")
+{
+  static_assert(!ex::sender<not_a_sender>);
+  static_assert(ex::sender<a_sender>);
+
+  static_assert(ex::sender_in<a_sender>);
+  static_assert(ex::sender_in<a_sender, ex::env<>>);
+
+  static_assert(ex::sender<non_constexpr_complsigs>);
+  static_assert(!ex::sender_in<non_constexpr_complsigs>);
+  static_assert(!ex::sender_in<non_constexpr_complsigs, ex::env<>>);
+
+  [[maybe_unused]] auto read_env = ex::read_env(ex::get_scheduler);
+  using read_env_t               = decltype(read_env);
+  static_assert(ex::sender<read_env_t>);
+  static_assert(!ex::sender_in<read_env_t>);
+  static_assert(!ex::sender_in<read_env_t, ex::env<>>);
+  static_assert(ex::sender_in<read_env_t, ex::prop<ex::get_scheduler_t, dummy_scheduler<>>>);
+}