Function pair_is_useful 

pub fn pair_is_useful(ctx: &RankContext<'_>, respect_aboard_path: bool) -> bool

Whether assigning ctx.car to ctx.stop is worth ranking.

Combines two checks every dispatch strategy needs at the top of its rank implementation:

1. Servability — capacity, full-load bypass, and the loading-phase boarding filter. A pair that can’t exit an aboard rider, board a waiter, or answer a rider-less hall call is a no-op move (and a zero-cost one when the car is already parked there) which would otherwise stall doors against unservable demand.
2. Path discipline (only when respect_aboard_path is true) — refuses pickups that would pull a car carrying routed riders off the direct path to every aboard rider’s destination. Without it, a stream of closer-destination hall calls can indefinitely preempt a farther aboard rider’s delivery (the “never reaches the passenger’s desired stop” loop).

Strategies with their own direction discipline (SCAN, LOOK, ETD, Destination) pass respect_aboard_path: false because their sweep/direction terms already rule out backtracks. Strategies without it (NearestCar, RSR) pass respect_aboard_path: true. Custom strategies should pass true unless they enforce direction discipline themselves.

Aboard riders without a published route (game-managed manual riders) don’t constrain the path — any pickup is trivially on-the-way for them, so the path check trivially passes when no aboard rider has a Route::current_destination.