As title suggests.

We gotta detect that in a placement handler to make sure the builder doesn't place colony blocks (through placeholder blocks) or scarecrow blocks outside of the colony boundaries.

ye thats what I was thinking, would avoid issues with other claims outside too

currently any action is restricted to be inside the boundaries, miner lumberjack etc. Should we really allow him to do anything outside of it?

Another solution would be not allowing a worker order to be placed where a partial part of it is outside of the colony border.