P0.1: restore immediate FAT evidence priority without UI starvation

MainWindow.IoTesting.cs

@@ -21,6 +21,7 @@ public partial class MainWindow
     private IoTestSessionController? _activeIoTestSessionController;
     private long _ioTestObservationSequence;
     private int? _pollingIntervalBeforeIoFat;
+    private int _ioFatEvidenceDrainDispatchActive;
 
     protected override void OnInitialized(EventArgs e)
     {
@@ -401,9 +402,37 @@ private IoTestSessionController CreateIoTestSession(IoTestProject project, strin
         => new(
             project,
             ResolveIoTestDevice,
-            action => Dispatcher.BeginInvoke(action, DispatcherPriority.Background),
+            DispatchIoFatEvidence,
             evidenceRoot);
 
+    private void DispatchIoFatEvidence(Action action)
+    {
+        ArgumentNullException.ThrowIfNull(action);
+
+        // A newly observed edge must reach the FAT evidence evaluator before normal UI
+        // background work, preserving the immediate TRUE-edge behavior from #150. When
+        // the bounded drain reschedules itself from inside the current callback, lower the
+        // continuation to Background so Render/Input are not starved by a large snapshot burst.
+        var priority = Volatile.Read(ref _ioFatEvidenceDrainDispatchActive) == 0
+            ? DispatcherPriority.DataBind
+            : DispatcherPriority.Background;
+
+        Dispatcher.BeginInvoke(
+            new Action(() =>
+            {
+                Interlocked.Increment(ref _ioFatEvidenceDrainDispatchActive);
+                try
+                {
+                    action();
+                }
+                finally
+                {
+                    Interlocked.Decrement(ref _ioFatEvidenceDrainDispatchActive);
+                }
+            }),
+            priority);
+    }
+
     private static string IoTestingProjectsRoot() => Path.Combine(
         Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),
         "ARSAS",

tests/ARSAS.Tests/IoTestFatP0RegressionTests.cs

@@ -29,7 +29,7 @@ public void FatConnect_ReadinessDoesNotBlockOnLegacyEightToEighteenSecondReportS
     }
 
     [Fact]
-    public void FatEvidenceDrain_YieldsUiAndStartupJournalUsesOneDurableBatch()
+    public void FatEvidenceDrain_PrioritizesFirstEdgeButYieldsContinuationAndBatchesStartupJournal()
     {
         var controller = Read("Services/IoTesting/IoTestSessionController.cs");
         var journal = Read("Services/IoTesting/IoTestEvidenceJournal.cs");
@@ -41,8 +41,16 @@ public void FatEvidenceDrain_YieldsUiAndStartupJournalUsesOneDurableBatch()
         Assert.Contains("AppendBatchRequired(startupEntries)", controller, StringComparison.Ordinal);
         Assert.Contains("AppendBatch(IEnumerable<IoTestJournalEntry> entries)", journal, StringComparison.Ordinal);
         Assert.Contains("FlushDurable();", journal, StringComparison.Ordinal);
+
+        // Preserve #150's immediate TRUE-edge scheduling while retaining P0's bounded
+        // drain/yield behavior for bursts. The first dispatch is DataBind; a reschedule
+        // requested while that drain is active is intentionally demoted to Background.
+        Assert.Contains("DispatchIoFatEvidence", main, StringComparison.Ordinal);
+        Assert.Contains("Volatile.Read(ref _ioFatEvidenceDrainDispatchActive) == 0", main, StringComparison.Ordinal);
+        Assert.Contains("DispatcherPriority.DataBind", main, StringComparison.Ordinal);
         Assert.Contains("DispatcherPriority.Background", main, StringComparison.Ordinal);
-        Assert.DoesNotContain("DispatcherPriority.DataBind", main, StringComparison.Ordinal);
+        Assert.Contains("Interlocked.Increment(ref _ioFatEvidenceDrainDispatchActive)", main, StringComparison.Ordinal);
+        Assert.Contains("Interlocked.Decrement(ref _ioFatEvidenceDrainDispatchActive)", main, StringComparison.Ordinal);
     }
 
     [Fact]