Add a minumum unit / weapon attack range

I suggest adding a minimum attack radius parameter to the engine for units. That is, a unit cannot attack a target or point that is closer than the minimum attack radius.

No file, test, or entry point is identified. Start by locating the engine's unit attack validation and existing attack-range parameters, then clarify how a minimum radius should apply to targets and points; done means units cannot attack either when the distance is below the configured minimum.

Written by the indexing model from the issue text.