fix(hub): derive Restart required from boot default vs running system

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.

modules/core/sovran-hub.nix

@@ -199,9 +199,11 @@ let
         echo "[ERROR] nixos-rebuild boot failed"
         RC=1
       elif ! readlink -f /nix/var/nix/profiles/system > "$GENERATION"; then
-        echo "[ERROR] update was built but its staged generation could not be recorded"
+        # The marker is informational only.  The Hub derives pending-reboot
+        # state from the NixOS system profile itself, so failing to record
+        # the marker must not fail an otherwise successful update.
+        echo "[WARNING] update succeeded but its staged generation could not be recorded"
         rm -f "$GENERATION"
-        RC=1
       fi
       echo ""
     fi