【后端接口】转盘抽奖：奖池概率与保底的兑付

---转盘的单次抽取
function Wheel.spin(actor)
    local player = class(actor)
    if not player then return false end
    if not checkitemw(actor, "转盘币", 1) then
        sendmsg(actor, 1, "需要 1 枚转盘币")
        return false
    end
    takeitem(actor, "转盘币", 1, 0, "转盘消耗")
    -- 奖池分层：金 1% / 紫 9% / 蓝 30% / 白 60%
    local r = math.random(1, 10000)
    local tier = (r <= 100 and 4) or (r <= 1000 and 3)
        or (r <= 4000 and 2) or 1
    local item = Wheel.pick(tier)
    giveitem(actor, item.name, item.count)
    Wheel.pityCount(actor, tier)
    sendmsg(actor, 1, "获得：" .. item.name .. " x" .. item.count)
    return item
end

---保底计数：40 抽必出金档
function Wheel.pityCount(actor, tier)
    if tier >= 4 then
        setplayvar(actor, "HUMAN", "WheelPity", 0, 1)
        return
    end
    local pity = tonumber(getplayvar(actor, 2, "WheelPity")) or 0
    pity = pity + 1
    if pity >= 40 then
        setplayvar(actor, "HUMAN", "WheelPity", 0, 1)
        giveitem(actor, "转盘大奖自选匣", 1)
        sendmsg(actor, 1, "保底触发！自选匣已发放")
    else
        setplayvar(actor, "HUMAN", "WheelPity", pity, 1)
        if pity == 35 then
            sendmsg(actor, 1, "距离保底还剩 5 抽！")
        end
    end
end