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_linux.go

@@ -24,12 +24,42 @@ func deleteRouteVia(cidr, gateway string) error {
 	return runCmd("ip", "route", "del", cidr, "via", gateway)
 }
 
+// --- IPv6 variants ---
+
+func addRoute6(cidr, iface string) error {
+	return runCmd("ip", "-6", "route", "add", cidr, "dev", iface)
+}
+
+func deleteRoute6(cidr, iface string) error {
+	return runCmd("ip", "-6", "route", "del", cidr, "dev", iface)
+}
+
+func addRouteVia6(cidr, gateway string) error {
+	return runCmd("ip", "-6", "route", "add", cidr, "via", gateway)
+}
+
+func deleteRouteVia6(cidr, gateway string) error {
+	return runCmd("ip", "-6", "route", "del", cidr, "via", gateway)
+}
+
 func defaultGateway() (string, error) {
 	out, err := exec.Command("ip", "route", "show", "default").Output()
 	if err != nil {
 		return "", err
 	}
-	fields := strings.Fields(strings.TrimSpace(string(out)))
+	return parseViaGateway(string(out))
+}
+
+func defaultGateway6() (string, error) {
+	out, err := exec.Command("ip", "-6", "route", "show", "default").Output()
+	if err != nil {
+		return "", err
+	}
+	return parseViaGateway(string(out))
+}
+
+func parseViaGateway(out string) (string, error) {
+	fields := strings.Fields(strings.TrimSpace(out))
 	for i, f := range fields {
 		if f == "via" && i+1 < len(fields) {
 			return fields[i+1], nil