Add iptables rules via a systemd transient unit to drop traffic
addressed to the bridge gateway IP from non-bridge interfaces.
The firewall manager waits for the transient unit to complete and
verifies success via D-Bus property change signals. On failure, the
system is marked unhealthy and host-network add-ons are prevented
from booting.
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>