feat: consolidate legacy parity migration baseline

src/MBN_STOCK_WEBVIEW.LegacyParityApp/MainWindow.xaml.cs

@@ -22,6 +22,7 @@ public sealed partial class MainWindow : Window
 
     private readonly CancellationTokenSource _lifetimeCancellation = new();
     private readonly SemaphoreSlim _intentGate = new(1, 1);
+    private readonly SemaphoreSlim _orderedPlayoutIntentGate = new(1, 1);
     private readonly PlayoutLaunchAuthorization _playoutLaunchAuthorization;
     private readonly WindowSubclassProcedure _windowSubclassProcedure;
     private readonly LegacyOperatorController _controller;
@@ -241,6 +242,7 @@ public sealed partial class MainWindow : Window
         await InitializeWebViewAsync();
         if (!_closing)
         {
+            StartDatabaseHealthMonitor();
             await ConnectPlayoutAsync(_lifetimeCancellation.Token);
         }
     }
@@ -380,15 +382,35 @@ public sealed partial class MainWindow : Window
     private async Task ProcessIntentAsync(LegacyUiIntent intent)
     {
         var gateEntered = false;
+        var orderedPlayoutIntentEntered = false;
+        var databaseGateEntered = false;
         var dispatchStarted = false;
         try
         {
             // MainForm performed database search synchronously. The busy snapshot
-            // freezes the Web controls, while this zero-timeout gate prevents an
-            // already-posted duplicate from growing an unbounded DB request queue.
-            gateEntered = await _intentGate.WaitAsync(
-                0,
-                _lifetimeCancellation.Token);
+            // freezes the Web controls. Playout commands must retain their order behind
+            // a preceding local row-selection intent, but only one may wait or execute
+            // at a time so a double post can never become a queued vendor command.
+            if (IsOrderedPlayoutIntent(intent))
+            {
+                orderedPlayoutIntentEntered = await _orderedPlayoutIntentGate.WaitAsync(
+                    0,
+                    _lifetimeCancellation.Token);
+                if (orderedPlayoutIntentEntered)
+                {
+                    await _intentGate.WaitAsync(_lifetimeCancellation.Token);
+                    gateEntered = true;
+                }
+            }
+            else
+            {
+                // Keep the zero-timeout admission used by database and edit intents;
+                // those requests must not build an unbounded queue behind slow I/O.
+                gateEntered = await _intentGate.WaitAsync(
+                    0,
+                    _lifetimeCancellation.Token);
+            }
+
             if (!gateEntered)
             {
                 PostState(IsNamedPlaylistModalIntent(intent)
@@ -514,6 +536,13 @@ public sealed partial class MainWindow : Window
                 PostState(_controller.Current);
             }
 
+            await AcquirePriorityDatabaseActivityAsync(_lifetimeCancellation.Token);
+            databaseGateEntered = true;
+            if (_closing)
+            {
+                return;
+            }
+
             dispatchStarted = true;
             var state = await HandleIntentAsync(intent, _lifetimeCancellation.Token);
             ObserveOperatorMutationQuarantine(state);
@@ -546,13 +575,26 @@ public sealed partial class MainWindow : Window
                 _intentBusy = false;
             }
 
+            if (databaseGateEntered)
+            {
+                _databaseActivityGate.Release();
+            }
+
             if (gateEntered)
             {
                 _intentGate.Release();
             }
+
+            if (orderedPlayoutIntentEntered)
+            {
+                _orderedPlayoutIntentGate.Release();
+            }
         }
     }
 
+    private static bool IsOrderedPlayoutIntent(LegacyUiIntent intent) => intent is
+        LegacyExecutePlayoutIntent or LegacyGateAPrepareIntent;
+
     private static bool IsNamedPlaylistModalIntent(LegacyUiIntent intent) =>
         intent is LegacyRefreshNamedPlaylistsIntent or
             LegacySelectNamedPlaylistIntent or
@@ -648,7 +690,8 @@ public sealed partial class MainWindow : Window
                 pointer.Shift,
                 pointer.X,
                 pointer.Y,
-                pointer.TimestampMilliseconds),
+                pointer.TimestampMilliseconds,
+                pointer.AllowAppend),
             LegacyCutKeyDownIntent key => _controller.CutKeyDown(
                 key.Key,
                 key.Control,
@@ -1347,6 +1390,7 @@ public sealed partial class MainWindow : Window
         DetachCloseConfirmation();
         DetachInteractionMetricsRefresh();
         _lifetimeCancellation.Cancel();
+        CancelActiveDatabaseHealthCheck();
         ShutdownPlayoutRuntime();
         ShutdownManualListsRuntime();
         DataQueryExecutor.Reset();