`Spring.SetSquareBuildingMask` has poor x/z interface

Bill of materials to be done before starting this ticket:
* awareness of #1579

----

It requires you to scale the x/z parameters by 16 (i.e. `Game.buildSquareSize`) manually. This requirement is obscure and mistake prone. You should be able to specify the actual x/z you want to affect without having to take internal implementation details into account.

Start with the Spring.SetSquareBuildingMask entry point and inspect how its x/z parameters are converted using Game.buildSquareSize. Read the awareness item in #1579 before deciding the intended coordinate behavior. Done means callers can provide the actual x/z area without manually scaling values by 16, with the relevant behavior verified by the project’s existing tests or reproduction path.

Written by the indexing model from the issue text.