[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/c2h/CMakeLists.txt

@@ -0,0 +1,65 @@
+cmake_minimum_required(VERSION 3.21)
+
+project(C2H LANGUAGES CXX CUDA)
+
+# when CCCL is used from the CTK, C2H does not need to be rebuilt when making local changes to CCCL (faster iteration)
+option(C2H_USE_CCCL_FROM_CTK "Use CCCL from the CTK in c2h." OFF)
+
+# Get dependencies.
+cccl_get_catch2()
+if (NOT C2H_USE_CCCL_FROM_CTK)
+  cccl_get_cccl()
+endif()
+cccl_get_cudatoolkit()
+
+set(has_curand OFF)
+if (TARGET CUDA::curand)
+  set(has_curand ON)
+endif()
+
+option(C2H_ENABLE_CURAND "Use CUDA CURAND library in c2h." ${has_curand})
+
+# Disable C2H_ENABLE_CURAND if cuRAND is unavailable.
+if (C2H_ENABLE_CURAND AND NOT has_curand)
+  message(
+    WARNING
+    "C2H_ENABLE_CURAND is requested, but CUDA::curand is unavailable. Disabling."
+  )
+  set(C2H_ENABLE_CURAND OFF)
+endif()
+
+add_library(
+  cccl.c2h
+  STATIC
+  generators.cu
+  generators_gen_values.cu
+  generators_uniform_offsets.cu
+  generators_vector.cu
+)
+target_compile_definitions(cccl.c2h PUBLIC CATCH_CONFIG_PREFIX_ALL)
+target_include_directories(cccl.c2h PUBLIC "${C2H_SOURCE_DIR}/include")
+target_link_libraries(
+  cccl.c2h
+  PUBLIC #
+    cccl.compiler_interface
+    Catch2::Catch2
+)
+if (NOT C2H_USE_CCCL_FROM_CTK)
+  target_link_libraries(cccl.c2h PUBLIC CCCL::CCCL)
+endif()
+cccl_configure_target(cccl.c2h DIALECT 17)
+
+if (C2H_ENABLE_CURAND)
+  target_link_libraries(cccl.c2h PRIVATE CUDA::curand)
+  target_compile_definitions(cccl.c2h PRIVATE C2H_HAS_CURAND=1)
+else()
+  target_compile_definitions(cccl.c2h PRIVATE C2H_HAS_CURAND=0)
+endif()
+
+set_target_properties(
+  cccl.c2h
+  PROPERTIES CXX_VISIBILITY_PRESET "default" CUDA_VISIBILITY_PRESET "default"
+)
+
+add_library(cccl.c2h.main OBJECT catch2_runner.cu catch2_runner_helper.cu)
+target_link_libraries(cccl.c2h.main PUBLIC cccl.c2h)