Machines whose install/update went UNSTABLE — the FB-7D50B0 (M4) class: a fresh install that then crash/restart-loops, so helper-backed tools (Contacts/Calendar/ Reminders) time out or fall back to a missing helper.

Two signals: • flapping_machines — tray_restarted events with flapping=true (the tray restarted again within 120s). DIRECT signal; needs a client >= the build that emits it. • unstable_install_machines — a burst of tool_errors within 20 min of an install event. INFERRED signal; works retroactively on existing data (would have flagged M4 ~12:17 today, before the manual problem report). Test/probe machines are excluded.