Raise atomic ranged events from the tree flattener

NodesInserted/NodesRemoved reported a multi-node change (a node plus its visible
descendants -- a contiguous run) as a sequence of single-item Add/Remove events,
but the underlying tree updates Count by the WHOLE run before they fire. A
consumer that reconciles the change one item at a time and re-indexes the source
mid-sequence then reads against a Count that has already dropped by the full run:
Avalonia's SelectionModel does exactly this (it re-reads SelectedItems while
handling each Remove), indexing past the end and throwing ArgumentOutOfRangeException
out of TreeFlattener's indexer whenever a selection was live during a bulk reshape
(e.g. toggling UseNestedNamespaceNodes). It only surfaced under full-suite timing,
which is why it read as flaky.

Raise one ranged event for the whole run instead, so the notification matches the
tree's state in a single step and Count/indices stay in agreement. The indexer
keeps throwing on genuine out-of-range access. Avalonia 12.4's virtualization and
selection handle ranged Add/Remove (every expand/collapse exercises it); full
headless suite is green.

ICSharpCode.ILSpyX/TreeView/TreeFlattener.cs

@@ -52,24 +52,28 @@ namespace ICSharpCode.ILSpyX.TreeView
 			CollectionChanged?.Invoke(this, e);
 		}
 
+		// A node plus its visible descendants form one contiguous run in the flattened list, and the
+		// underlying tree's Count is updated by the WHOLE run before these fire. Raise a single ranged
+		// event so the notification matches the tree: Count and indices stay in agreement for a
+		// consumer that reconciles the change in one step. Per-node events instead leave the source
+		// already fully resized while the consumer is still mid-sequence, so one that re-indexes during
+		// reconciliation (Avalonia's SelectionModel re-reading SelectedItems) reads past the end.
 		public void NodesInserted(int index, IEnumerable<SharpTreeNode> nodes)
 		{
 			if (!includeRoot)
 				index--;
-			foreach (SharpTreeNode node in nodes)
-			{
-				RaiseCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, node, index++));
-			}
+			IList list = nodes as IList ?? new List<SharpTreeNode>(nodes);
+			if (list.Count > 0)
+				RaiseCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, list, index));
 		}
 
 		public void NodesRemoved(int index, IEnumerable<SharpTreeNode> nodes)
 		{
 			if (!includeRoot)
 				index--;
-			foreach (SharpTreeNode node in nodes)
-			{
-				RaiseCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Remove, node, index));
-			}
+			IList list = nodes as IList ?? new List<SharpTreeNode>(nodes);
+			if (list.Count > 0)
+				RaiseCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Remove, list, index));
 		}
 
 		public void Stop()