Add a way to set void state

Attaching a unit to another can put it in the void state:
https://github.com/beyond-all-reason/spring/blob/41c58746c30843fe946332f1a2f868f27c430149/rts/Sim/Units/Unit.cpp#L2523

The request is to add a way to do it standalone without the need to attach it to another unit.

Start by inspecting rts/Sim/Units/Unit.cpp around line 2523 and trace how attaching a unit causes the void state. Identify the existing state transition and the public entry point that could invoke it independently; done means a unit can enter the void state without being attached to another unit.

Written by the indexing model from the issue text.