Roebuck's Law 5: Mechanical Coupling with Reaction Mass Accounting

This methods preprint distinguishes internal deformation from net thrust. Internal hydraulic, elastic, or mechanical actuators may redistribute momentum without expelling reaction mass, but an isolated center of mass cannot accelerate. Net momentum change requires external force or mass/field momentum flux. Version 2 keeps force, momentum rate, power, radiation pressure, and nuisance-force accounting dimensionally distinct. No reactionless thrust or measured device performance is claimed.