holder

Whatever holds the zone, or null when it is free.

Typed to ZoneHolderIfc rather than to GuidedTransporter because exclusive occupation of space is not a vehicle's privilege: a closed aisle or a pedestrian crossing denies a zone to traffic in exactly the same way, and the engine has no reason to care which it is. Only the parts that genuinely reason about vehicles -- a transporter's own run of covered zones, and the wait-for graph's outgoing edges -- ask what kind of holder this is.