docs(zero-swift): one line per paragraph in DESIGN.md (#50)

Unwraps the prose that was hard-wrapped at 80 columns. Text, lists,
tables and code blocks are unchanged.

Amp-Thread-ID: https://ampcode.com/threads/T-01a17664-8fd0-76be-8fd5-75ac6e7aa0d8

Co-authored-by: Amp <amp@ampcode.com>

Author: tmm