[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/stf/cpp/wait_token.cu

@@ -0,0 +1,60 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of CUDASTF 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) 2022-2024 NVIDIA CORPORATION & AFFILIATES.
+//
+//===----------------------------------------------------------------------===//
+
+//! \file
+//!
+//! \brief Test ctx.wait() on a token: a blocking, value-less synchronization
+
+#include <cuda/experimental/stf.cuh>
+
+#include <type_traits>
+
+using namespace cuda::experimental::stf;
+
+__global__ void set_value(int* p, int v)
+{
+  *p = v;
+}
+
+template <typename context_t>
+void run()
+{
+  context_t ctx;
+
+  // Externally owned buffer: STF only schedules around it, it never owns it.
+  int* d_val = nullptr;
+  cuda_safe_call(cudaMalloc(&d_val, sizeof(int)));
+
+  auto tok = ctx.token();
+
+  ctx.task(tok.write())->*[=](cudaStream_t s) {
+    set_value<<<1, 1, 0, s>>>(d_val, 42);
+  };
+
+  // wait(token) has no value to materialize: it must return void and only
+  // block the host until the token's producing work has completed.
+  static_assert(::std::is_void_v<decltype(ctx.wait(tok))>, "wait(token) must return void");
+  ctx.wait(tok);
+
+  int h_val = 0;
+  cuda_safe_call(cudaMemcpy(&h_val, d_val, sizeof(int), cudaMemcpyDeviceToHost));
+  _CCCL_ASSERT(h_val == 42, "wait(token) did not synchronize the producing task");
+
+  ctx.finalize();
+  cuda_safe_call(cudaFree(d_val));
+}
+
+int main()
+{
+  run<stream_ctx>();
+  run<graph_ctx>();
+  run<context>();
+  run<stackable_ctx>();
+}