Will fix actor stacking bug - UnloadCargo and Cargo.SpawnPassengers

Requires improvements to sub-cell logic. (I'm working on it.)
Supersedes #6060 and #6063
Fixes UnloadCargo and Cargo.SpawnPassengers by queuing a slightly smarter nudge w/ others that
have the same CenterPosition.
Use `TraitOrDefault<>` here.  `Trait<>` throws when a trait isn't available, `TraitOrDefault<>` returns `null`