What sort of subject line would be appropriate here?
I think you are overthinking this. Something simple like "Resignation letter" works fine. You want to be clear about the content, just like any other email you send.
Though, I'm not sure why you are doing so in email vs in-person or phone. See this questionthis question for why. It's generally best to communicate bad news in person/phone over email (regardless of what, whether resignation or about a system going down).