Units of measure — Conversion inside a class only — never across one

Units convert freely within a class. Across classes, conversion is refused rather than guessed. This is the whole safety property. Turning a mass into a length depends entirely on what is being measured, and a silent 1:1 fallback is exactly how a figure ends up wrong by three orders of magnitude with nothing raised. If you genuinely need to cross classes — kilograms per metre of a specific product — that factor belongs on the record, not in the unit table.