拖拽换位：格子拖拽的交换规则

-- 错误：松手直接覆盖，目标格物品凭空消失
function onDrop(from, to)
    bag[to] = bag[from]
    redrawSlot(to)
end

local function swapSlot(from, to)
    if from == to then
        redrawSlot(from)
        return
    end
    if lockedSlot(to) then
        redrawSlot(from)
        showTip("该格不可放置")
        return
    end
    bag[from], bag[to] = bag[to], bag[from]
    redrawSlot(from)
    redrawSlot(to)
    saveBag(bag)
end