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

tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/InstanceActorNativeAlarmTests.cs

@@ -100,6 +100,7 @@ public class InstanceActorNativeAlarmTests : TestKit, IDisposable
     {
         var dcl = CreateTestProbe();
         var actor = CreateInstanceActorWithDcl("inst", ConfigWithNativeSource("inst"), dcl.Ref);
+        dcl.ExpectMsg<SubscribeAlarmsRequest>();
 
         // No live condition is emitted: the configured "Pressure" binding is quiet.
         actor.Tell(new SubscribeDebugViewRequest("inst", "c"));
@@ -121,6 +122,7 @@ public class InstanceActorNativeAlarmTests : TestKit, IDisposable
     {
         var dcl = CreateTestProbe();
         var actor = CreateInstanceActorWithDcl("inst", ConfigWithNativeSource("inst"), dcl.Ref);
+        dcl.ExpectMsg<SubscribeAlarmsRequest>();
 
         // The NativeAlarmActor emits a live condition stamped with the binding's
         // canonical name (DV-1: NativeSourceCanonicalName), so the binding is "active".