Refusing dimension travel

entity_travel_to_dimension runs before a server entity moves to the target level. Returning Flow::Cancel refuses that entity's travel. Passengers are asked recursively: refusing one leaves it dismounted in the source level, with its own passengers, without refusing the vehicle's transition. Discarding an entity through its handle during the callback also prevents its travel. Callbacks may change riding relationships. An original passenger edge is restored only if the checked mount operation still permits it; otherwise the passenger stays behind, without overwriting any new vehicle chosen by a callback. A falling block ticks outside the level's entity list, so its handle is not accessible during this callback.