Remove salt scuffing

[Roomba]

Salt piles disappear pretty much whenever anyone walks over them, which makes it impossible to keep any significantly-traversed area salted for a long time. Wraiths go from practically unkillable to functionally immortal, because they have much more room to maneuver and a much lower chance of slipping up. As a result salting large areas only works if one guy does it and everyone else holes up in their rooms, which doesn't make for good interaction,

Yes, I know the trick is hiding salt piles under tables and obstacles and such, but tricks like that only work if the wraith has an incentive to stay off the beaten path, which it doesn't when any roaming assistant can clear the path for it.