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

pkg/config/config.go

@@ -2,6 +2,7 @@ package config
 
 import (
 	"fmt"
+	"net"
 	"os"
 	"time"
 
@@ -33,6 +34,8 @@ type VRRPInstance struct {
 	NotifyBackup    string   `yaml:"notify_backup"`
 	NotifyFault     string   `yaml:"notify_fault"`
 	TrackScripts    []string `yaml:"track_scripts"`
+	UnicastSrcIP    string   `yaml:"unicast_src_ip"`
+	UnicastPeers    []string `yaml:"unicast_peers"`
 }
 
 type HealthChecker struct {
@@ -84,6 +87,20 @@ func validate(cfg *Config) error {
 		if len(vrrp.VirtualIPs) == 0 {
 			return fmt.Errorf("vrrp_instances[%d].virtual_ips cannot be empty", i)
 		}
+
+		if len(vrrp.UnicastPeers) > 0 {
+			for j, peer := range vrrp.UnicastPeers {
+				if ip := net.ParseIP(peer); ip == nil || ip.To4() == nil {
+					return fmt.Errorf("vrrp_instances[%d].unicast_peers[%d] is not a valid IPv4 address: %s", i, j, peer)
+				}
+			}
+		}
+
+		if vrrp.UnicastSrcIP != "" {
+			if ip := net.ParseIP(vrrp.UnicastSrcIP); ip == nil || ip.To4() == nil {
+				return fmt.Errorf("vrrp_instances[%d].unicast_src_ip is not a valid IPv4 address: %s", i, vrrp.UnicastSrcIP)
+			}
+		}
 	}
 
 	return nil