Upgradeable yardmap type 'b' doesn't work for spawned units

To recreate:
1. `/give armmex`
2. `/give armack`
3. Try to give a build order for an T2 mex on top of the T1 mex

The build placement grid will show that the T2 mex is not buildable. If the T1 mex is built rather than spawned, this problem does not exist.

Reproduce the issue with `/give armmex` and `/give armack`, then compare T2 mex placement over a spawned T1 mex with placement over a normally built T1 mex. Start by tracing the upgradeable yardmap type `b` handling and build-placement grid checks; done means both cases allow the T2 mex build order.

Written by the indexing model from the issue text.