UpdateChecker now derives the latest release and a releasesBehind
count from one releases-list call. When the installed version is
forceUpdateThreshold (3) or more stable releases behind, the update
dialog becomes mandatory: barrier and back-press are blocked, the
later/don't-remind actions are hidden, backing out of the installer
returns to the dialog, and the check bypasses the user's
check-for-updates opt-out. Prereleases never count toward the
threshold.