Add balancer fallbackTag

https://github.com/2dust/v2rayN/issues/9401

v2rayN/ServiceLib/Global.cs

@@ -55,7 +55,7 @@ public class Global
     public const string DnsOutboundTag = "dns";
     public const string DnsTag = "dns-module";
     public const string DirectDnsTag = "direct-dns";
-    public const string BalancerTagSuffix = "-round";
+    public const string BalancerTagSuffix = "-balancer";
     public const string StreamSecurity = "tls";
     public const string StreamSecurityReality = "reality";
     public const string Loopback = "127.0.0.1";

v2rayN/ServiceLib/Models/CoreConfigs/V2rayConfig.cs

@@ -276,6 +276,7 @@ public class BalancersItem4Ray
     public List<string>? selector { get; set; }
     public BalancersStrategy4Ray? strategy { get; set; }
     public string? tag { get; set; }
+    public string? fallbackTag { get; set; }
 }
 
 public class BalancersStrategy4Ray

v2rayN/ServiceLib/Services/CoreConfig/V2ray/V2rayBalancerService.cs

@@ -104,6 +104,7 @@ public partial class CoreConfigV2rayService
                 },
             },
             tag = balancerTag,
+            fallbackTag = multipleLoad == EMultipleLoad.Fallback ? Global.DirectTag : null,
         };
         _coreConfig.routing.balancers ??= new();
         _coreConfig.routing.balancers.Add(balancer);