Build an elevator system

• A building with a configurable number of floors and elevators; each elevator has a current floor, direction (UP, DOWN, IDLE), door state, a capacity, and a set of stops
• External requests (a person on a floor presses UP or DOWN) and internal requests (a person inside presses a destination floor)
• A dispatch strategy assigns each external request to one elevator, for example the nearest elevator already moving toward the request in the same direction, otherwise the nearest idle one, otherwise the one that will be free soonest
• Each elevator uses the SCAN (elevator) algorithm: keep going in the current direction serving stops, then reverse when there are no more stops ahead; an idle elevator with no stops stays where it is
• Time is advanced by a tick() call (one floor per tick, plus door time at stops), so behaviour is deterministic and testable
• Invalid input is rejected (unknown floor, UP from the top floor); duplicate requests for the same stop are merged
• A state snapshot shows each elevator's floor, direction and pending stops; the system never sends an elevator in the wrong direction for a request it has accepted

Only after every requirement works.

• Capacity and overload handling, skipping pickups when full
• Maintenance mode that removes an elevator from dispatch and reassigns its requests
• Peak-time strategies, such as returning idle elevators to the lobby