Authorship when PhD project given to new post-doc?
First things first: I fully agree with Buffy and whoever else that this sounds like a very dodgy situation. A supervisor reassigning work and then 'not wanting to discuss authorship at this stage' takes an enormous amount of agency away from you as a PhD student. That has to be demotivating and frustrating. You could indeed look into transferring to another supervisor.

If you want to stay with this group, the project sounds like a co-first authorship to me. If you've done 3/4 of the project already, that seems fair even if the postdoc will optimise some of your results. Since taking it up with your supervisor wasn't really fruitful, talk to the postdoc and make sure they see this project as a shared first publication as well. After that, stay involved by discussing results, , asking what you can do to help, helping with troubleshooting etc. Make sure you know what stage the project is at and especially when writing starts, help with that. Obviously if you can improve on results, that's a bonus, but just staying involved should come a long way.