Save flags somehow

Flags are lost when closing the document. It would be useful to persist them, but where?

* In the file's text (somehow)?
* As a file attribute (xattr)?
* In a side-file?
* Someplace invisible, tied to the file's path or contents?

Start by reviewing how the macOS app represents documents and flags, then compare the persistence options listed in the issue: embedded text, xattrs, a side-file, or storage tied to the path or contents. The issue does not name files, tests, or an entry point, and completion criteria are not defined; a maintainer decision is needed before implementation.

Written by the indexing model from the issue text.