Fast prototypes are useful only when they answer clear questions. Before the first build, define user context, measurable requirements, physical environment, interfaces, manufacturing assumptions, verification method, and the decision the prototype should support.
NASA's systems engineering guidance treats good requirements as the baseline for verification and validation. Prototype research also shows that physical artifacts need metadata: who made it, why, when, where, with which material and process settings, and what was learned.
RHXY Plan is designed to turn early uncertainty into a traceable brief that can feed Sim and Render workflows later.