I really, really don't think your engine is seizing/ seized. It hasn't even started, or built up enough friction to seize. It's gotta be in your start clutch, or wait..!. Check this:
Is your variator binding against the subframe? If you forgot the little spacer first, the variator will bind against the subframe and you won't be able to turn it over.
But I don't believe there is any way your piston is seizing in the cylinder. That requires heat expansion, and yours hasn't even fired. Even with completely dry assembly the piston isn't going to seize by turning the engine over by hand or pedal.