feat: add VRRP unicast peer support for macvlan and restricted network environments

- Add unicast_src_ip and unicast_peers config fields
- NewUnicastSocket binds to source IP for proper packet reception
- SendTo() sends VRRP advertisements directly to peer IPs
- Receive loop filters packets from non-peer sources in unicast mode
- Backward compatible: multicast mode used when unicast_peers is empty
- Add unicast config example (etc/config.unicast.yaml)
- Update README with unicast documentation and examples

internal/vrrp/manager.go

@@ -36,6 +36,8 @@ func (m *Manager) LoadFromConfig(cfg *config.Config) error {
 			vrrpCfg.AuthType,
 			vrrpCfg.AuthPass,
 			vrrpCfg.TrackScripts,
+			vrrpCfg.UnicastSrcIP,
+			vrrpCfg.UnicastPeers,
 			m.log,
 		)
 		if err != nil {