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

src/ZB.MOM.WW.ScadaBridge.HealthMonitoring/IClusterNodeProvider.cs

@@ -13,9 +13,13 @@ public interface IClusterNodeProvider
     IReadOnlyList<NodeStatus> GetClusterNodes();
 
     /// <summary>
-    /// True when this node is currently the cluster leader (Primary) for the
-    /// provider's role scope. Used by the central report loop to decide which
-    /// node should generate the "central" health report.
+    /// True when this node is currently the oldest Up member for the provider's
+    /// role scope — the node the ClusterSingletonManager hosts singletons on.
+    /// This is the canonical product "active node" check (review 01): the site
+    /// event-log purge gate, the central self-report loop, and (via plan 02) the
+    /// S&amp;F delivery gate all consume it. It is deliberately NOT the cluster
+    /// leader — leadership is address-ordered and diverges from singleton
+    /// placement once the original first node restarts and rejoins.
     /// </summary>
     bool SelfIsPrimary { get; }
 }