generate/rate_comprehension/solver.py: solve_rate computes the one unknown — quantity=rate×time (always integer), rate=quantity÷time, time=quantity÷rate (exact division or REFUSE non_integer_solution, never rounds). Confirms compound-unit composition via the R3a algebra (defensive; a RateProblem's units compose by construction). answer_unit gives the answer's unit. No floats.
Ties to the gold: solves all 6 solved fixtures to gold; refuses both solver_refuses with non_integer_solution. Meaningful-fail: 100÷3 refuses (not 33), 99÷3=33. Off-serving. 5 tests.
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.
