What do you "call" yourself if mentioning a side project in your CV?

A title only means something in the context of an organization. Titles refer to boxes on org charts and rows in a pay scales spreadsheet.

Look at all of the titles in your job history and re-word them into descriptive labels if necessary. "Senior Programmer" works across organizations, "Code Wrangler II" does not. Titles don't have any consistent meaning in the programming field.

If you want to call out work done outside of an organization describe it in clear language: open source contributor, side project, hobby project, etc.

Some employers may see side projects as a negative. They might wonder if you will just take a job to finance a startup, or work on your side projects on their time or with their resources. I suggest thinking through that possibility and only presenting side projects if they have some relevance to the job, and you can assure the potential employer you won't use their time to do your own thing, or quit as soon as you get a few paying customers for the side gig.

unless its somethin really huge, leave it off your c.v. if the conversation goes well and you feel its ok/useful to, you might mention it there.

In my experience side projects are the least important part of a resume. Employers just aren’t interested in over achievers.

In my case I would prominently feature my side projects to showcase my capabilities, discoveries, and potential. Nobody cares.

What’s important is your employment history, years of relevant experience, and percentage match to the criteria of the current roll. It’s not about how good you are but well you fit. The better of a developer you are might actually work against you. If you have a lot of side projects, like I do, and have invested substantial effort to advance your capabilities, as I have, I am sure you will find that supremely depressing.

Always remember the goal is employment, not craftsmanship.

a side project is a creative work.