generate/rate_comprehension/units.py: BaseUnit + RateUnit (numerator/denominator) + the three single-rate compositions — rate_times_time (mile/hour × hour = mile; refuses mile/hour × minute), rate_from_quantity_over_time (mile ÷ hour = mile/hour), time_from_quantity_over_rate (mile ÷ mile/hour = hour; refuses dollar ÷ mile/hour). A non-composing op raises UnitError (the wrong=0 dimensional gate), never fabricates a unit.
Deliberately tiny: bare-name units, no generic dimensional universe, no semantic unit-class typing, no unit conversion, no compound-of-compound. Fresh off-serving organ (generate/rate_comprehension/). 10 tests incl. all three refusals + consistent composition.
