#1246 build: vmm.hpp declares size_t via <cstddef> (CUDA 13 headers dropped the transitive include)

描述

Building with CUDA 13.3 fails on `vmm.hpp`: `size_t` was only visible through a header that CUDA 13 no longer pulls in transitively. One line; the full engine build is green afterwards (nvcc 13.3, sm_86).