> you have no responsibility to them other than being professional and seeing out your notice.
Do you really have that as a responsibility? Would the company give you notice or fire you on the day?
Cultural/juridistion miscommunication here :)
In some places, and varying by contract your mandated to give a notice and the same applies to employer.
In my jurisdiction I need to give 2 months If I'm employed there for more than 2 years (>=2).
You've made the mistake of believing that you are "part of" the startup. You are just "employed by" the startup - you are not in charge and you can't rely on them for anything. The other side to this equation is you can just quit very easily and get a job somewhere else - you have no responsibility to them other than being professional and seeing out your notice.
Get a job in a bar for a few months, then go back to programming. You're obviously talented - you'll easily get another job.