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.
