Dispose source in Remove_Middle perf test cleanup

The CleanUp lambda in Remove_Middle_ComparerIsSlowerThanNoComparer only
disposed the FilteredList wrapper; the underlying ObservableList stayed
alive across measurement iterations. Dispose the source too so each
iteration starts from a clean state.

https://claude.ai/code/session_0133w4Rt2GPd43rjC6UG5Dfg

Author: claude

Tests/Observable/Performance/FilteredListPerformanceTests.cs

@@ -92,7 +92,11 @@ public void Remove_Middle_ComparerIsSlowerThanNoComparer()
                     for (var i = 0; i < size; i++) source.Add(i);
                     filtered = new FilteredList<int>(source);
                 })
-                .CleanUp(() => filtered.Dispose())
+                .CleanUp(() =>
+                {
+                    filtered.Dispose();
+                    source.Dispose();
+                })
                 .SampleGroup(new SampleGroup("Remove_Middle_NoComparer", SampleUnit.Microsecond))
                 .WarmupCount(WarmupCount)
                 .MeasurementCount(MeasurementCount)
@@ -105,7 +109,11 @@ public void Remove_Middle_ComparerIsSlowerThanNoComparer()
                     for (var i = 0; i < size; i++) source.Add(i);
                     filtered = new FilteredList<int>(source, Comparer<int>.Default);
                 })
-                .CleanUp(() => filtered.Dispose())
+                .CleanUp(() =>
+                {
+                    filtered.Dispose();
+                    source.Dispose();
+                })
                 .SampleGroup(new SampleGroup("Remove_Middle_Comparer", SampleUnit.Microsecond))
                 .WarmupCount(WarmupCount)
                 .MeasurementCount(MeasurementCount)