change only basic shields, any future change in production target within city turn resets this value - current edithand.c effect
Likely, we should change correspondingly before_change_shields. There is a field last_turns_shield_surplus in editing protocol but it is currently not used.
Noticed in #42681, relates to all versions:
Likely, we should change correspondingly before_change_shields. There is a field last_turns_shield_surplus in editing protocol but it is currently not used.