fix(communication): drop IsConfiguredPlaceholder rows in StreamRelayActor before gRPC pack

Placeholder AlarmStateChanged rows are a DebugView snapshot-only concept emitted
by InstanceActor.BuildAlarmStatesSnapshot; they are never a real alarm transition.
Their timestamp may be DateTimeOffset.MinValue (the Protobuf Timestamp lower boundary),
which can throw when packed via Timestamp.FromDateTimeOffset.

Added early-return guard at the top of HandleAlarmStateChanged before any timestamp
pack or channel write. Updated the existing NativeBindingLinkage round-trip test to
use a real (non-placeholder) native alarm; added DropsAlarmStateChanged_WhenIsConfiguredPlaceholder
to assert placeholders are silently dropped (15/15 pass).

src/ZB.MOM.WW.ScadaBridge.Communication/Actors/StreamRelayActor.cs

@@ -56,6 +56,15 @@ public class StreamRelayActor : ReceiveActor
 
     private void HandleAlarmStateChanged(AlarmStateChanged msg)
     {
+        // Placeholder rows (IsConfiguredPlaceholder) are a Debug View snapshot-only
+        // concept emitted by InstanceActor.BuildAlarmStatesSnapshot — they are never a
+        // real alarm transition and must not be relayed to the live gRPC stream (their
+        // timestamp may be DateTimeOffset.MinValue, the Protobuf Timestamp boundary).
+        if (msg.IsConfiguredPlaceholder)
+        {
+            return;
+        }
+
         var protoEvent = new SiteStreamEvent
         {
             CorrelationId = _correlationId,