[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_task_scheduler.cu

@@ -0,0 +1,94 @@
+//===----------------------------------------------------------------------===//
+//
+// 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/checked_receiver.cuh" // IWYU pragma: keep
+#include "common/dummy_scheduler.cuh" // IWYU pragma: keep
+#include "common/error_scheduler.cuh" // IWYU pragma: keep
+#include "common/stopped_scheduler.cuh" // IWYU pragma: keep
+#include "common/utility.cuh" // IWYU pragma: keep
+
+namespace ex = cuda::experimental::execution;
+
+namespace
+{
+C2H_TEST("simple task_scheduler test", "[scheduler][task_scheduler]")
+{
+  ex::task_scheduler sched{dummy_scheduler{}};
+  STATIC_CHECK(ex::scheduler<decltype(sched)>);
+  auto sndr = sched.schedule();
+  STATIC_CHECK(ex::sender<decltype(sndr)>);
+  auto op = ex::connect(cuda::std::move(sndr), checked_value_receiver{});
+  ex::start(op);
+  // The receiver checks that it's called
+}
+
+C2H_TEST("task_scheduler starts work on the correct execution context", "[scheduler][task_scheduler]")
+{
+  ex::thread_context ctx;
+  ex::task_scheduler sched{ctx.get_scheduler()};
+  auto sndr  = ex::starts_on(sched, ex::just() | ex::then([] {
+                                     return ::std::this_thread::get_id();
+                                   }));
+  auto [tid] = ex::sync_wait(cuda::std::move(sndr)).value();
+  CHECK(tid == ctx.get_id());
+}
+
+#if !_CCCL_HOST_COMPILATION()
+static __device__ bool g_called = false;
+#else
+static bool g_called = false;
+#endif
+
+template <class Sndr>
+struct protect : private Sndr
+{
+  using sender_concept = ex::sender_t;
+  _CCCL_HOST_DEVICE_API explicit protect(Sndr sndr)
+      : Sndr{cuda::std::move(sndr)}
+  {}
+  using Sndr::connect;
+  using Sndr::get_completion_signatures;
+  using Sndr::get_env;
+};
+
+struct test_domain
+{
+  _CCCL_TEMPLATE(class Sndr, class Env)
+  _CCCL_REQUIRES(ex::sender_for<Sndr, ex::bulk_chunked_t>)
+  _CCCL_HOST_DEVICE_API auto transform_sender(ex::set_value_t, Sndr sndr, const Env&) const
+  {
+    return ex::then(protect{cuda::std::move(sndr)}, []() noexcept {
+      g_called = true;
+    });
+  }
+};
+
+C2H_TEST("bulk_unchunked dispatches correctly through task_scheduler", "[scheduler][task_scheduler]")
+{
+  ex::task_scheduler sched{dummy_scheduler<test_domain>{}};
+  auto sndr  = ex::on(sched, ex::just(-1) | ex::bulk_chunked(ex::par_unseq, 100, [](int, int, int&) {}));
+  g_called   = false;
+  auto [val] = ex::sync_wait(cuda::std::move(sndr)).value();
+  CHECK(val == -1);
+  CHECK(g_called);
+}
+
+C2H_TEST("bulk dispatches correctly through task_scheduler", "[scheduler][task_scheduler]")
+{
+  ex::task_scheduler sched{dummy_scheduler<test_domain>{}};
+  auto sndr  = ex::on(sched, ex::just(-1) | ex::bulk(ex::par_unseq, 100, [](int, int&) {}));
+  g_called   = false;
+  auto [val] = ex::sync_wait(cuda::std::move(sndr)).value();
+  CHECK(val == -1);
+  CHECK(g_called);
+}
+} // namespace