NixOS already knows whether a reboot is pending: /nix/var/nix/profiles/
system vs /run/current-system. Marker files only the Hub's own updater
wrote desynced for terminal-updated machines (and markers from older
updaters could never clear), pinning the badge on forever. Reconcile
REBOOT_REQUIRED against live state on every read; the stale marker
self-heals to IDLE. The .generation marker write is now informational.