I don’t think any software based protection will be able to shut off the triac in time. The short will happen instantly and there won’t be time for the microcontoller to respond. I’ve seen some commercial sprinkler controllers using a huge 10 ohm resistor for short protection. It works, but it’s not going to allow users to open multiple valves at the same time. So in case you don’t open more than 2 valves at a time, you can put a 10 ohm, 25 watt resistor in line with the common wire to server as short protection.