Todo

Condensed from TODO.md at build time: 5 open item(s), 0 completed.

• decide how long to keep the replaced file: `update` keeps it as `<name>.previous`, which for FreeCAD is another 820 MB. A retention rule (delete on the next successful update, or on request) would be worth having.
• use a zsync client for updates when one is installed, so an update transfers a delta instead of the whole file. `zsync`, `zsync2` and `appimageupdatetool` are all absent here, so `update` downloads in full today; the check already prints the size.
• decide what to do about the version a file name states: `version_of_appimage` reads OrcaSlicer's `…V2.4.2_62a8fff…` as `2.4`, which is what an update check would compare against a release tag.
• improve the human-oriented documents in `documents/`.
• read the tool-universe sources in `documents/02-tool-universe.md`.