feat(desktop): invoke exact external-app menu commands once (#140)

* feat(desktop): invoke exact menu commands once

* fix(desktop): explain native menu child refusals

* chore(desktop): compare self menu read affinity

* fix(desktop): prepare self menu reads on the main actor

* chore(desktop): clarify self menu read threading

* chore(desktop): rebase menu patch on image writes

* fix(desktop): refuse unsupported self menu commands

* fix(desktop): preserve menu patch after item-open integration