Add stricter class-visible loadout slot checks

[nosoop]

bitsSlotVisibility controls which items are valid for slots, but we only check this at item parse time.

Item assignments from before this change will persist (and at the moment, will be applied), but there is no way to access them. We should simply ignore those slots; maybe log a warning or provide an SQL script to clean up unused items.

We should also validate the inventory slot against the item data.