setup: coldcore crashes on public.txt file transfer

This is on Windows, I'm not sure if it is reproducible on Mac/Linux.

Steps to reproduce:

- Start coldcore (before having loaded public.txt in)
- Get to the point where it says it's waiting for public.txt
- Load public.txt into coldcore folder

I think coldcore is trying to access public.txt before it is fully written to the folder. I don't have a copy of the error message (because coldcore cleared the output the next time I ran it) but I believe it was a permissions error, presumably because it was in the process of being written to disk.

Start by reproducing the Windows flow: launch coldcore, wait for the public.txt prompt, and transfer public.txt into the coldcore folder. Inspect how the transfer is detected and read; done means coldcore no longer crashes when the file is still being written and continues setup successfully.

Written by the indexing model from the issue text.