Rounding race split times so they add up to the goal time

Race split tables must use whole seconds, but naively rounding each segment causes the totals to drift from the goal time. For example, a 4:00:00 marathon with kilometre splits can end 11 seconds short when each segment is rounded independently. The proposed fix is to round the cumulative elapsed time at each checkpoint and derive each segment as the difference between rounded values. This keeps every checkpoint within half a second of the exact plan and forces the final row to match the goal. The approach also guards against zero-length segments and handles the partial final distance correctly.