feat: 新增 IPv6 双栈支持

- 协议: InitMessage 增加 ip6/prefix6/server_ip6 字段(omitempty,向后兼容)
- TUN: Device 接口新增 ConfigureIPv6, macOS 用 ifconfig inet6, Linux 用 ip -6 addr add
- 路由: full 模式自动添加 ::/1+8000::/1, v6 服务器旁路 /128, resolveHosts 双栈解析
- 会话: setupTUN 解析 v6 字段并配置 TUN+路由, connectOnce 传 v6 给 stats/日志
- 统计: SetConnected(ip,ip6), Snapshot.AssignedIP6
- DB: connection_logs 增加 assigned_ip6 列 + migrateV3 幂等迁移
- UI: 状态卡片新增独立 IPv6 行, 始终可见(未获取显示 IPv6: —)
- 文档: 同步 client-development.md 至服务端 IPv6 版本

internal/route/route_darwin.go

@@ -9,7 +9,8 @@ import (
 )
 
 // addRoute adds a route via a network interface (macOS route command).
-//   route add -inet -net <cidr> -interface <iface>
+//
+//	route add -inet -net <cidr> -interface <iface>
 func addRoute(cidr, iface string) error {
 	return runRoute("add", "-inet", "-net", cidr, "-interface", iface)
 }
@@ -29,13 +30,44 @@ func deleteRouteVia(cidr, gateway string) error {
 	return runRoute("delete", "-inet", "-net", cidr, gateway)
 }
 
-// defaultGateway returns the current default gateway IP.
+// --- IPv6 variants ---
+
+func addRoute6(cidr, iface string) error {
+	return runRoute("add", "-inet6", "-net", cidr, "-interface", iface)
+}
+
+func deleteRoute6(cidr, iface string) error {
+	return runRoute("delete", "-inet6", "-net", cidr, "-interface", iface)
+}
+
+func addRouteVia6(cidr, gateway string) error {
+	return runRoute("add", "-inet6", "-net", cidr, gateway)
+}
+
+func deleteRouteVia6(cidr, gateway string) error {
+	return runRoute("delete", "-inet6", "-net", cidr, gateway)
+}
+
+// defaultGateway returns the current IPv4 default gateway IP.
 func defaultGateway() (string, error) {
 	out, err := exec.Command("route", "-n", "get", "default").Output()
 	if err != nil {
 		return "", err
 	}
-	for _, line := range strings.Split(string(out), "\n") {
+	return parseGateway(string(out))
+}
+
+// defaultGateway6 returns the current IPv6 default gateway IP.
+func defaultGateway6() (string, error) {
+	out, err := exec.Command("route", "-n", "get", "-inet6", "default").Output()
+	if err != nil {
+		return "", err
+	}
+	return parseGateway(string(out))
+}
+
+func parseGateway(out string) (string, error) {
+	for _, line := range strings.Split(out, "\n") {
 		line = strings.TrimSpace(line)
 		if strings.HasPrefix(line, "gateway:") {
 			parts := strings.SplitN(line, ":", 2)