Track active Desktop runtime preparation in the _desktop hook, expose installing status through Desktop session status, and have the Desktop frontend poll with a neutral install message instead of showing missing dependencies while packages are still being installed after an update.
Add the built-in _desktop plugin as the owner of Xpra/Xfce lifecycle, /desktop route installation, Desktop state, session APIs, surface registration, and the linux-desktop skill. Leave _office with explicit compatibility facades and self-update delegates so users coming from 1.10 through 1.13 keep their runtime cleanup path.
