指引气泡：任务目标的头顶指引

-- 错误：固定屏幕坐标，转视角气泡空飘
bubble:setPosition(300, 200)

local function onFrameFollow(bubble, target)
    local sx, sy = target:getWorldPos()
    if offScreen(sx, sy) then
        showArrow(bubble, sx, sy)
        bubble:setVisible(false)
    else
        bubble:setVisible(true)
        bubble:setPosition(sx, sy + 60)
    end
end