[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/examples/stf/pi.cu

@@ -0,0 +1,52 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 Approximate pi using Monte Carlo method
+ *
+ */
+
+#include <cuda/experimental/stf.cuh>
+
+#include <curand_kernel.h>
+#include <stdio.h>
+
+using namespace cuda::experimental::stf;
+
+int main(int, char**)
+{
+  context ctx;
+  auto lsum = ctx.logical_data(shape_of<scalar_view<size_t>>());
+
+  size_t N = 1000000;
+
+  ctx.parallel_for(box(N), lsum.reduce(reducer::sum<size_t>{}))->*[] __device__(size_t i, auto& sum) {
+    curandState local_state;
+    curand_init(1234, i, 0, &local_state);
+    double x = curand_uniform_double(&local_state); // Random x in [0, 1)
+    double y = curand_uniform_double(&local_state); // Random y in [0, 1)
+    // Count (x,y) coordinates which are within the unit circle
+    if (x * x + y * y <= 1.0)
+    {
+      sum++;
+    }
+  };
+
+  // We get the ratio of "shots" within the unit circle and the total number of
+  // "shots". The surface of the quarter of unit circle [0, 1) x [0, 1) is pi/4
+  auto res      = ctx.wait(lsum);
+  double pi_val = (4.0 * res) / N;
+
+  ctx.finalize();
+
+  _CCCL_ASSERT(fabs(pi_val - 3.1415) < 0.1, "Invalid result");
+}