Option A (text-faithful): RateProblem gains time_unit (the duration's ORIGINAL unit; time stays int; defaults to the rate denominator). The SOLVER converts via convert_time -> exact Fraction -> int-or-refuse(non_integer); Fraction never leaves the solver, no floats. The reader accepts a convertible (minute<->hour) duration mismatch (keeps original time_unit); a non-convertible one still refuses rate_unit_mismatch. Signature includes time_unit ('30 minutes' != '30 hours').
Gold: r3-09 flips reader_refuses -> SOLVED (60 mph for 30 min = 30 mile, exact 1/2 hour; distractor 1800 rejected); new r3-13 non-convertible (3 gallons) stays rate_unit_mismatch. R3 gold 13/13; reader 9/0/0 -> answers 7/0/6 (setup_correct 8->9). Failure-family: rate_unit_mismatch now non-convertible-only -> still unsupported_rate_duration proposal surface; router-hygiene invariant stays green; R1 7/0/3, R2 10/0/0 unchanged. 119-test smoke green incl. invariants. Off-serving, no float path.
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.
