fix: preserve prepared workflow state

src/MBN_STOCK_WEBVIEW.Core/Playout/Scenes/LegacyPlayoutWorkflow.cs

@@ -74,10 +74,12 @@ public sealed class LegacyPlayoutWorkflow
     private readonly IPlayoutEngine _engine;
     private readonly ILegacySceneCueProvider _cueProvider;
     private readonly SemaphoreSlim _gate = new(1, 1);
+    private readonly object _engineStatusSync = new();
     private IReadOnlyList<LegacyPlaylistEntry> _playlist = [];
     private ActivePage? _prepared;
     private ActivePage? _onAir;
     private int _engineClearRequested;
+    private long _lastObservedEngineStatusSequence = long.MinValue;
 
     public LegacyPlayoutWorkflow(
         IPlayoutEngine engine,
@@ -98,13 +100,27 @@ public sealed class LegacyPlayoutWorkflow
     public void ObserveEngineStatus(PlayoutStatus status)
     {
         ArgumentNullException.ThrowIfNull(status);
-        if (!string.IsNullOrWhiteSpace(status.PreparedSceneName) ||
-            !string.IsNullOrWhiteSpace(status.OnAirSceneName))
+        var hasActiveScene = !string.IsNullOrWhiteSpace(status.PreparedSceneName) ||
+            !string.IsNullOrWhiteSpace(status.OnAirSceneName);
+        lock (_engineStatusSync)
+        {
+            if (status.Sequence <= _lastObservedEngineStatusSequence)
+            {
+                return;
+            }
+
+            _lastObservedEngineStatusSequence = status.Sequence;
+            // Availability/OnHello updates can report no scene while PREPARE still owns
+            // the workflow gate. A later authoritative PREPARED/PROGRAM status must
+            // cancel that deferred clear before the command publishes its page metadata.
+            Interlocked.Exchange(ref _engineClearRequested, hasActiveScene ? 0 : 1);
+        }
+
+        if (hasActiveScene)
         {
             return;
         }
 
-        Interlocked.Exchange(ref _engineClearRequested, 1);
         if (!_gate.Wait(0))
         {
             return;
@@ -538,15 +554,18 @@ public sealed class LegacyPlayoutWorkflow
 
     private void ApplyRequestedEngineClear()
     {
-        if (Interlocked.Exchange(ref _engineClearRequested, 0) == 0)
+        lock (_engineStatusSync)
         {
-            return;
-        }
+            if (Interlocked.Exchange(ref _engineClearRequested, 0) == 0)
+            {
+                return;
+            }
 
-        _prepared = null;
-        _onAir = null;
-        _playlist = [];
-        State = EmptyState;
+            _prepared = null;
+            _onAir = null;
+            _playlist = [];
+            State = EmptyState;
+        }
     }
 
     private static readonly LegacyPlayoutWorkflowState EmptyState = new(