关注降价：关注商品的降价提醒

local function watch(actor, goods)
    WATCH[goods] = actor            -- 只记录关注，不比对价格
end

local WATCH = {}
local function watch(actor, goods, price)
    WATCH[goods] = WATCH[goods] or {}
    WATCH[goods][actor] = price     -- 登记关注价
end
local function refresh(goods, newPrice)
    for a, oldPrice in pairs(WATCH[goods] or {}) do
        if newPrice < oldPrice then
            local p = getplayerbyname(a)
            if p then
                sendmsg(p, 1, goods .. " 降价至 " .. newPrice)
                WATCH[goods][a] = newPrice   -- 更新基线防重复
            end
        end
    end
end