Check `unitdef.maxthisunit` robustness

Claims that you can work around it and exceed the limit by giving multiple factories a build order at the same time: https://github.com/beyond-all-reason/Beyond-All-Reason/pull/5895

Start by reviewing the linked pull request #5895 and locating the code path that enforces unitdef.maxthisunit. Reproduce the reported case with multiple factories receiving build orders simultaneously, then verify that the limit remains enforced or document the confirmed failure.

Written by the indexing model from the issue text.