The boiling water may shock the enzymes right by were you are pouring the boiling water but there will be plenty more in the rest of the mash to convert your starch to sugar. With that said the temperature difference for the enzymes are not an all or nothing transition. It is more like at 152F enzyme A is at 80% activity while enzyme B is at 100%. Raise it to 156F and enzyme A drops to 20% while enzyme B is now at 80%. If you let it cool back down to 152F your enzymes will move to A - 50% and B - 90%. As you can see they will not get back up to their starting activity but will still be working. Never the less 1 qt of boiling water is not going to cause any noticeable difference for the whole mash. I would not worry about it.