Typed ABIs (#[repr(C)] SnapshotPayload, FillPayload) backed by pinned_mem::MappedF32Buffer. write_volatile is the only hot-path CPU->GPU pathway; GPU reads via device_ptr() with zero HtoD. Tests pass on local sm_86 (3/3): snapshot round-trip, fill round-trip, pre-allocation invariant (device pointer stable across writes). Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>