Resolve all high-severity architecture review findings

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

@@ -52,6 +52,8 @@ public class NativeAlarmActor : ReceiveActor
 
     /// <summary>Buffer accumulating a snapshot replay until <see cref="AlarmTransitionKind.SnapshotComplete"/>.</summary>
     private readonly Dictionary<string, NativeAlarmTransition> _snapshotBuffer = new();
+    private Guid? _snapshotId;
+    private Guid? _lastCompletedSnapshotId;
 
     /// <summary>
     /// Pending mirror upserts coalesced by source reference (P4): under an alarm storm each
@@ -135,6 +137,9 @@ public class NativeAlarmActor : ReceiveActor
     /// <inheritdoc />
     protected override void PostStop()
     {
+        _dclManager.Tell(new UnsubscribeAlarmsRequest(
+            Guid.NewGuid().ToString("N"), _instanceName, _source.ConnectionName,
+            _source.SourceReference, DateTimeOffset.UtcNow), Self);
         _retryTimer?.Cancel();
         _flushTimer?.Cancel();
         // Best-effort final flush of any coalesced upserts so a graceful stop does not lose the
@@ -227,14 +232,29 @@ public class NativeAlarmActor : ReceiveActor
     private void HandleTransition(NativeAlarmTransitionUpdate update)
     {
         var t = update.Transition;
+        if (t.SnapshotSourceReference is { } scope && scope != _source.SourceReference)
+            return;
         switch (t.Kind)
         {
+            case AlarmTransitionKind.SnapshotStart:
+                if (t.SnapshotId is null || t.SnapshotId == _lastCompletedSnapshotId || t.SnapshotId == _snapshotId)
+                    return;
+                _snapshotId = t.SnapshotId;
+                _snapshotBuffer.Clear();
+                break;
+
             case AlarmTransitionKind.Snapshot:
+                if (t.SnapshotId != _snapshotId)
+                    return;
                 _snapshotBuffer[t.SourceReference] = t;
                 break;
 
             case AlarmTransitionKind.SnapshotComplete:
+                if (t.SnapshotId != _snapshotId || (t.SnapshotId is not null && t.SnapshotId == _lastCompletedSnapshotId))
+                    return;
                 ApplySnapshotSwap();
+                _lastCompletedSnapshotId = t.SnapshotId;
+                _snapshotId = null;
                 break;
 
             default:
@@ -310,6 +330,10 @@ public class NativeAlarmActor : ReceiveActor
 
     private void HandleSourceUnavailable(NativeAlarmSourceUnavailable msg)
     {
+        // An interrupted refresh is not a snapshot. Never merge its partial rows
+        // into the next connection's replay.
+        _snapshotBuffer.Clear();
+        _snapshotId = null;
         // Keep last-known conditions (uncertain) rather than clearing them; the
         // reconnect snapshot reconciles state. No emit — avoids a flap.
         _logger.LogWarning(