Add custom effect mesh to air staging

It needs a prop spawned on it so that we can apply a see through shader on it for a force field effect. that prop also needs cleaning up if the unit dies, and so on.

purely visual effect, but would be good to get it right since then we can also propagate it to other meshes like the build mesh on the factory arms and so on.

Start by locating the air staging unit definition and the existing prop or mesh effect lifecycle in the mod. Determine how a spawned prop can receive the see-through shader and how unit death or removal is handled. Done means the force-field effect appears on air staging and its prop is cleaned up reliably, with the approach suitable for other meshes.

Written by the indexing model from the issue text.