[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_trampoline_scheduler.cu

@@ -0,0 +1,97 @@
+//===----------------------------------------------------------------------===//
+//
+// 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/retry.cuh"
+#include "testing.cuh"
+
+namespace ex = ::cuda::experimental::execution;
+
+#if !_CCCL_DEVICE_COMPILATION()
+
+namespace
+{
+struct try_again
+{};
+
+class fails_alot
+{
+  template <class Receiver>
+  struct operation;
+
+public:
+  using sender_concept = ex::sender_t;
+
+  fails_alot() = default;
+
+  __host__ fails_alot(fails_alot&& other) noexcept
+      : counter_(std::move(other.counter_))
+  {}
+
+  __host__ fails_alot(const fails_alot& other) noexcept
+      : counter_(other.counter_)
+  {}
+
+  template <class Receiver>
+  [[nodiscard]] auto connect(Receiver rcvr) const noexcept -> operation<Receiver>
+  {
+    return operation<Receiver>{static_cast<Receiver&&>(rcvr), --*counter_};
+  }
+
+  template <class...>
+  static _CCCL_CONSTEVAL auto get_completion_signatures() noexcept
+  {
+    return ex::completion_signatures<ex::set_value_t(), ex::set_error_t(try_again)>{};
+  }
+
+private:
+  template <class Receiver>
+  struct operation
+  {
+    void start() & noexcept
+    {
+      if (counter_ == 0)
+      {
+        ex::set_value(static_cast<Receiver&&>(rcvr_));
+      }
+      else
+      {
+        ex::set_error(static_cast<Receiver&&>(rcvr_), try_again{});
+      }
+    }
+
+    Receiver rcvr_;
+    int counter_;
+  };
+
+  std::shared_ptr<int> counter_ = std::make_shared<int>(1'000'000);
+};
+
+// #if defined(REQUIRE_TERMINATE)
+// // For some reason, when compiling with nvc++, the forked process dies with SIGSEGV
+// // but the error code returned from ::wait reports success, so this test fails.
+// TEST_CASE("running deeply recursing algo blows the stack", "[schedulers][trampoline_scheduler]") {
+
+//   auto recurse_deeply = retry(fails_alot{});
+//   REQUIRE_TERMINATE([&] { sync_wait(std::move(recurse_deeply)); });
+// }
+// #endif
+
+TEST_CASE("running deeply recursing algo on trampoline_scheduler doesn't blow the stack",
+          "[scheduler][trampoline_scheduler]")
+{
+  ex::trampoline_scheduler sched;
+  auto recurse_deeply = retry(ex::on(sched, fails_alot{}));
+  ex::sync_wait(std::move(recurse_deeply));
+}
+} // namespace
+
+#endif // !_CCCL_DEVICE_COMPILATION()