Write always stamped DateLastModified with the save time. That is right
for an ordinary save, but a converter carrying another system's date
cannot keep it: every PEP nest imported into the server recorded the
import time as its modified date.
PreserveDateLastModified (init, default false) writes the nest's own
date instead; every existing caller keeps stamping.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
