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/relay/config/config.go

@@ -1,49 +1,18 @@
 package config
 
 import (
-	"context"
 	"encoding/json"
 	"errors"
-	"net"
 	"os"
 	"path/filepath"
 	"strings"
 	"time"
 
-	"github.com/Rain-kl/Wavelet/pkg/geoip"
-	"github.com/Rain-kl/Wavelet/pkg/geoip/iputil"
+	edgeconfig "github.com/Rain-kl/Wavelet/internal/apps/edge/config"
+	"github.com/Rain-kl/Wavelet/internal/apps/edge/nodeip"
 )
 
-type MillisecondDuration time.Duration
-
-func (d *MillisecondDuration) UnmarshalJSON(b []byte) error {
-	var v interface{}
-	if err := json.Unmarshal(b, &v); err != nil {
-		return err
-	}
-	switch value := v.(type) {
-	case float64:
-		*d = MillisecondDuration(time.Duration(value) * time.Millisecond)
-		return nil
-	case string:
-		duration, err := time.ParseDuration(value)
-		if err != nil {
-			return err
-		}
-		*d = MillisecondDuration(duration)
-		return nil
-	default:
-		return errors.New("invalid duration format")
-	}
-}
-
-func (d MillisecondDuration) Duration() time.Duration {
-	return time.Duration(d)
-}
-
-func (d MillisecondDuration) String() string {
-	return time.Duration(d).String()
-}
+type MillisecondDuration = edgeconfig.MillisecondDuration
 
 type Config struct {
 	ServerURL         string              `json:"server_url"`
@@ -130,7 +99,7 @@ func applyDefaults(cfg *Config, baseDir string) {
 		cfg.NodeName = strings.TrimSpace(host)
 	}
 	if cfg.NodeIP == "" {
-		cfg.NodeIP = detectNodeIP()
+		cfg.NodeIP = nodeip.Detect()
 	}
 	if cfg.StatePath == "" {
 		cfg.StatePath = filepath.Join(cfg.DataDir, "relay-state.json")
@@ -180,56 +149,4 @@ func (cfg *Config) Save() error {
 	return os.WriteFile(cfg.configPath, data, 0o644)
 }
 
-func detectNodeIP() string {
-	if ip := detectOutboundNodeIP(); ip != "" {
-		return ip
-	}
-	return detectLocalNodeIP()
-}
 
-func detectOutboundNodeIP() string {
-	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
-	defer cancel()
-	ip, err := geoip.GetOutboundIP(ctx)
-	if err != nil || ip == nil {
-		return ""
-	}
-	return ip.String()
-}
-
-func detectLocalNodeIP() 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
-}