Installers write a .fresh-install breadcrumb on fresh installs only and
delete stale breadcrumbs on upgrade; none of them writes login items or
registry Run keys. Windows NSIS detects upgrades via the uninstall
registry entry or an existing installed executable; the macOS pkg via the
previous pkgutil receipt; Linux deb/rpm via the standard postinstall
arguments. Post-update GUI relaunches (macOS open, Linux
ui-post-install.sh) pass --post-update so the first-run autostart default
cannot fire on updates.