perf: reduce diagnostics and queued dispatch allocations in 2.3.2

Tests/ShrinkEventBusDeferredDiagnosticsTests.cs

@@ -73,6 +73,35 @@ namespace ShrinkEventBus.Tests
             Assert.AreEqual(1, secondObserverCalls);
         }
 
+        [Test]
+        public void ObserveDiagnostics_MutationsApplyToNextDispatch()
+        {
+            var calls = new List<string>();
+            IDisposable second = null;
+            IDisposable added = null;
+            using var first = EventBus.ObserveDiagnostics(_ =>
+            {
+                calls.Add("first");
+                second.Dispose();
+                if (added == null)
+                    added = EventBus.ObserveDiagnostics(__ => calls.Add("added"));
+            });
+            second = EventBus.ObserveDiagnostics(_ => calls.Add("second"));
+            try
+            {
+                EventBus.NotifyDetailedPosted(default);
+                CollectionAssert.AreEqual(new[] { "first", "second" }, calls);
+                calls.Clear();
+                EventBus.NotifyDetailedPosted(default);
+                CollectionAssert.AreEqual(new[] { "first", "added" }, calls);
+            }
+            finally
+            {
+                second.Dispose();
+                added?.Dispose();
+            }
+        }
+
         [Test]
         public void PostDeferred_DoesNotDispatchBeforePump()
         {