属性加点：点数分配的加减与重置

-- 错误：只有加法入口，多点的点数永不回头
function addPoint(p, attr)
    setVar(p, "attr_" .. attr,
        (getVar(p, "attr_" .. attr) or 0) + 1)
end

local function addPoint(p, attr, n)
    local used = getDraft("usedPts") or 0
    local total = getDraft("totalPts") or 0
    local cur = getDraft("attr_" .. attr) or 0
    if used + n > total or cur + n < 0 then
        showTip("点数不足或越界")
        return
    end
    writeDraft("usedPts", used + n)
    writeDraft("attr_" .. attr, cur + n)
    previewAttr(attr, cur + n)
end