Pull the "date -> leave/RTT exercise year" formula out of EmployeeRttPaymentProcessor, EmployeeRttSummaryProvider and EmployeeLeaveSummaryProvider into a single App\Service\Exercise\ExerciseYearResolver. Forfait flag is parameterised so the leave (calendar year) and RTT (Juin N-1 -> Mai N) variants share the same implementation. Pure refactor, no behavioural change.