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.
