- Flush routes per dead gateway instead of waiting for entire group to die
- Add iperf3 bandwidth (success=0 → dead) as third health signal
- Ping threshold: >=50% loss is now dead (was 100%)
- State tracking per gateway (.state/GROUP/gw-TAG/)
- GRE detection: use link UP instead of RX bytes delta (fixes false
  disconnected status on idle tunnels)
- Timer interval: 20s (was 1min)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>