fix(host): unify active-node on oldest-member semantics — purge/self-report/labels now track the singleton host

src/ZB.MOM.WW.ScadaBridge.Host/Health/ActiveNodeGate.cs

@@ -9,9 +9,12 @@ namespace ZB.MOM.WW.ScadaBridge.Host.Health;
 /// <see cref="IActiveNodeGate"/> backed by the running Akka.NET cluster.
 ///
 /// The inbound API is "Central cluster only (active node)" — a standby central
-/// node must not execute method scripts or <c>Route.To()</c> calls. This gate
-/// mirrors the leadership check in <see cref="ActiveNodeHealthCheck"/> (the
-/// node is the cluster leader, <see cref="MemberStatus.Up"/>), so
+/// node must not execute method scripts or <c>Route.To()</c> calls. Active =
+/// the oldest Up member (the singleton host), evaluated by
+/// <see cref="ClusterActivityEvaluator.SelfIsOldest(Cluster, string?)"/> — NOT
+/// the cluster leader, whose address-ordered definition diverges from singleton
+/// placement after a restart (review 01 [High]). This is the same evaluator
+/// backing <see cref="OldestNodeActiveHealthCheck"/>, so
 /// <see cref="InboundApiEndpointFilter"/> can return HTTP 503 on a standby.
 ///
 /// Registered only in the Central-role branch of <c>Program.cs</c>. The gate
@@ -42,12 +45,7 @@ public sealed class ActiveNodeGate : IActiveNodeGate
                 return false;
 
             var cluster = Cluster.Get(system);
-            var self = cluster.SelfMember;
-            if (self.Status != MemberStatus.Up)
-                return false;
-
-            var leader = cluster.State.Leader;
-            return leader != null && leader == self.Address;
+            return ClusterActivityEvaluator.SelfIsOldest(cluster);
         }
     }
 }