0011. Explicit preloads instead of carried-over trips

Decision. PyTraf no longer carries unfinished trips from one period into the next. assign_periods, Period and the carry-over computation are removed. In their place, assign takes optional link_preload and turn_preload: fixed volumes in car units that are not routed but add to the volume the delay functions see. In Rust the same is done by wrapping a delay function in Preloaded.

Why. - The carried share, \(q \min(\tau, T)/T\), depends on path travel times. In a static model these come from volume-delay functions with no queues, and they are inflated on overloaded links. More traffic is then carried, which congests the next period further: the feature amplifies a known weakness of the static model. - With periods of two or three hours the carried share is a few percent and changes little. It matters only for short periods relative to trip times, where a dynamic model is the right tool. - The approximations (no rerouting, no second carry-over, spread over the whole next period) were hard to defend together, and static tools commonly run periods independently. - A fixed preload is the general, standard mechanism (background traffic, buses, through traffic) and needed no new solver code.

Consequences. - Several periods are separate assign calls, with capacities scaled to each period's length by the caller. - A user who wants carry-over can compute it from path_table() and pass it as the next preload, as an explicit modelling choice. - The Beckmann objective with a preload counts only the area above the preload, so it is still reported for a single class with PCE 1.