【数据存储】存档版本降级读取：新旧字段兼容的回退方案

local FIELD_VER = {
    title = { v2 = "TITLE_V2", v1 = "TITLE_V1", def = "" },
    mount = { v2 = "MOUNT_V2", v1 = "MOUNT_ID", def = 0 },
}
local function readField(actor, name)
    local player = class(actor)
    local def = FIELD_VER[name]
    if def == nil then return nil end
    local v = getplayvar(actor, 2, def.v2)
    if v ~= nil and v ~= "" then
        return v
    end
    v = getplayvar(actor, 2, def.v1)
    if v ~= nil and v ~= "" then
        return v
    end
    return def.def
end

local function upgradeSave(actor)
    local player = class(actor)
    local ver = tonumber(getplayvar(actor, 2, "SAVE_VER") or "1")
    while ver < 2 do
        if ver == 1 then
            local old = getplayvar(actor, 2, "TITLE_V1")
            if old ~= nil and old ~= "" then
                setplayvar(actor, "HUMAN", "TITLE_V2", old, 1)
            end
        end
        ver = ver + 1
    end
    setplayvar(actor, "HUMAN", "SAVE_VER", ver, 1)
end