perf(mqtt): suppress unchanged-value republish fan-out

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt/MqttSubscriptionManager.cs

@@ -1,3 +1,4 @@
+using System.Collections;
 using System.Collections.Concurrent;
 using System.Collections.Frozen;
 using System.Globalization;
@@ -748,8 +749,39 @@ public sealed class MqttSubscriptionManager
                 snapshot.StatusCode);
         }
 
+        // Unchanged-value suppression. Every poll driver gets this dedupe for free at the edge from
+        // PollGroupEngine; MQTT is push-based, so a broker that timer-republishes an identical payload
+        // would otherwise re-trigger the ENTIRE downstream fan-out (actor hop → dependency mux → OPC UA
+        // sink write) per message. The cache is still freshened — a read must serve the current
+        // timestamp — and only the notification is skipped. "Cached but not raised" is already a valid
+        // state on this path (see Raise: an authored-but-unsubscribed tag does exactly that).
+        var prior = Values.Read(def.Name);
+        var unchanged = prior.StatusCode == snapshot.StatusCode && !ValuesAreDifferent(prior.Value, snapshot.Value);
+
         Values.Update(def.Name, snapshot);
-        Raise(def.Name, snapshot);
+        if (!unchanged) Raise(def.Name, snapshot);
+    }
+
+    /// <summary>
+    ///     Returns <c>true</c> when <paramref name="previous"/> and <paramref name="current"/> represent
+    ///     different values. Mirrors <c>PollGroupEngine.ValuesAreDifferent</c> — deliberately duplicated
+    ///     rather than shared, because that one is private to the poll-loop engine this push-based driver
+    ///     does not run through.
+    /// </summary>
+    /// <remarks>
+    ///     Arrays are compared structurally: a JSON array payload decodes to a fresh array instance on
+    ///     every message, so reference equality would report every republish as a change and suppress
+    ///     nothing at all for exactly the tags whose payloads are largest.
+    /// </remarks>
+    /// <param name="previous">The last cached value for the reference.</param>
+    /// <param name="current">The value just extracted from the inbound message.</param>
+    /// <returns><c>true</c> when the two differ.</returns>
+    private static bool ValuesAreDifferent(object? previous, object? current)
+    {
+        if (previous is Array prevArr && current is Array currArr)
+            return !StructuralComparisons.StructuralEqualityComparer.Equals(prevArr, currArr);
+
+        return !Equals(previous, current);
     }
 
     /// <summary>