gilflorida2023/simplesieve ? reverse-engineered prompt

Build me a fast Go command line tool that generates prime numbers using a parallel segmented sieve, and make it work like the examples in the README.

I want to be able to ask for either the first N primes or all primes up to a limit, choose a wheel size, and optionally just count the primes instead of printing them. If no output file is given, it should print a SHA 256 hash by default, and it should also support writing the primes to a file or to stdout with a dash. Add a progress option too so I can see it working on big runs.

Please make it feel polished and practical, with sensible help text, clear flags, and good performance for large inputs. If you need to check current Go docs or best practices for parsing flags and fast output, feel free to look them up online.