refactor(edge): 抽取边缘运行时共享包并完成 Phase 3 重构

- 新增 internal/apps/edge/，三组件改为薄包装，删除 3000+ 行重复代码
- Agent 心跳周期下沉至 heartbeat/cycle.go
- 协议类型迁入 pkg/protocol/agent.go
- 补充设计文档与 changelog

internal/apps/edge/nodeip/nodeip.go

@@ -0,0 +1,69 @@
+package nodeip
+
+import (
+	"context"
+	"net"
+	"time"
+
+	"github.com/Rain-kl/Wavelet/pkg/geoip"
+	"github.com/Rain-kl/Wavelet/pkg/geoip/iputil"
+)
+
+var (
+	LookupOutboundIP = geoip.GetOutboundIP
+	LookupLocalIP    = DetectLocal
+)
+
+func Detect() string {
+	if ip := detectOutbound(); ip != "" {
+		return ip
+	}
+	return LookupLocalIP()
+}
+
+func detectOutbound() string {
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	ip, err := LookupOutboundIP(ctx)
+	if err != nil || ip == nil {
+		return ""
+	}
+	return ip.String()
+}
+
+func DetectLocal() string {
+	interfaces, err := net.Interfaces()
+	if err != nil {
+		return ""
+	}
+	bestIP := ""
+	bestPriority := -1
+	for _, iface := range interfaces {
+		if iface.Flags&net.FlagUp == 0 || iface.Flags&net.FlagLoopback != 0 {
+			continue
+		}
+		addrs, err := iface.Addrs()
+		if err != nil {
+			continue
+		}
+		for _, addr := range addrs {
+			ipNet, ok := addr.(*net.IPNet)
+			if !ok || ipNet.IP == nil || ipNet.IP.IsLoopback() {
+				continue
+			}
+			ipv4 := ipNet.IP.To4()
+			if ipv4 == nil {
+				continue
+			}
+			priority := iputil.Score(ipv4)
+			if priority > bestPriority {
+				bestIP = ipv4.String()
+				bestPriority = priority
+			}
+			if bestPriority == 2 {
+				return bestIP
+			}
+		}
+	}
+	return bestIP
+}