Unwanted days are shared evenly across people

A roster that satisfies every hard rule can still be the thing that starts a dispute. These rules measure how unevenly the disliked things land, weekends, nights, overtime, awkward shift transitions, and push the solver toward flattening that distribution. They are mostly soft rules, because fairness is a quantity to be optimised rather than a line to be crossed, and because a unit that is short-staffed has to trade some of it away knowingly.