Full DOM fetcher captures content before it stabilizes, causing blinks #1256

Some tracked documents blink: consecutive recorded versions flip back and forth between two states of the same page, with no corresponding edit from the service. This adds noise to the versions history.

Example, two consecutive versions about 12 hours apart on 2026-06-28:

• OpenTermsArchive/user-rights-versions@49b3b70 (10:38 UTC) removes a section and adds another.
• OpenTermsArchive/user-rights-versions@9bd8b9e (22:38 UTC) reverts it: the removed section reappears and the added one is removed again.

This likely comes from the full DOM fetcher (executeClientScripts): the page is captured as soon as the selected elements hold content, before client-side rendering has settled. On pages that render or reorder their content progressively, consecutive runs capture different intermediate states, which surface as versions flipping back and forth.

A possible solution would be to wait for the selected content to settle before capturing by observing the selected elements with a MutationObserver and capturing only once no mutation has been recorded for a short quiet period.