背包整理：一键归类的排序规则

-- 错误：带洞的格子表直接排序，踩 nil 报错
table.sort(slots, function(a, b)
    return a.quality > b.quality
end)

local function tidyBag(slots)
    local items = {}
    for _, it in pairs(slots) do
        if it ~= nil then
            items[#items + 1] = it
        end
    end
    table.sort(items, function(a, b)
        if a.quality ~= b.quality then
            return a.quality > b.quality
        end
        return a.type < b.type
    end)
    for i = 1, #slots do
        slots[i] = items[i]
    end
end