Secret Key Exchange activity enhancements

- [ ] Defer second half of calculations until first half is complete (NEXT button?)
- [ ] Animate swapping of public keys
- [ ] Add a note about discrete logarithm problem
- [ ] Explain why they get the same number at the end

Start by locating the Secret Key Exchange activity in the JavaScript application and run it to understand its current calculation flow and public-key interaction. Done means the four listed enhancements are implemented: staged calculations, animated key swapping, a discrete-logarithm note, and an explanation of why both participants finish with the same number.

Written by the indexing model from the issue text.