fix(DV-2): clarify placeholder comment, stable MinValue timestamp, drain DCL probe in new tests

- Replace placeholder-loop comment with the double-render guard explanation
- Use _alarmTimestamps.GetValueOrDefault(binding, DateTimeOffset.MinValue) so the
  placeholder timestamp is stable/idempotent across snapshot calls (was UtcNow)
- Add dcl.ExpectMsg<SubscribeAlarmsRequest>() drain in Snapshot_QuietNativeBinding_EmitsPlaceholder
  and Snapshot_NativeBindingWithLiveCondition_NoPlaceholder to consume the DCL message
  the NativeAlarmActor sends at startup

src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/InstanceActor.cs

@@ -1117,9 +1117,11 @@ public class InstanceActor : ReceiveActor
                 _alarmTimestamps.GetValueOrDefault(name, DateTimeOffset.UtcNow)));
         }
 
-        // Native source bindings with no live condition: emit a placeholder so the
-        // Debug View tree shows the configured binding node even when quiet. A binding
-        // is "quiet" when no retained event carries its canonical name.
+        // A native binding is "already represented" if any retained event carries its
+        // canonical name — it renders in the Debug View via those condition rows. The
+        // placeholder below covers ONLY bindings with zero retained events (truly quiet
+        // since startup). Do NOT narrow this to State==Active: a binding holding a
+        // retained Normal/resolved condition would then be rendered twice.
         var liveBindings = _latestAlarmEvents.Values
             .Where(e => !string.IsNullOrEmpty(e.NativeSourceCanonicalName))
             .Select(e => e.NativeSourceCanonicalName)
@@ -1130,7 +1132,8 @@ public class InstanceActor : ReceiveActor
             if (liveBindings.Contains(binding)) continue;
 
             states.Add(new AlarmStateChanged(
-                _instanceUniqueName, binding, AlarmState.Normal, 0, DateTimeOffset.UtcNow)
+                _instanceUniqueName, binding, AlarmState.Normal, 0,
+                _alarmTimestamps.GetValueOrDefault(binding, DateTimeOffset.MinValue))
             {
                 Kind = _nativeAlarmKinds.GetValueOrDefault(binding, AlarmKind.NativeOpcUa),
                 NativeSourceCanonicalName = binding,