Validate negative startIndex in ObservableList.RemoveRange

Throw ArgumentOutOfRangeException before taking the lock, matching the
existing count guard, so callers get a deterministic exception instead
of an IndexOutOfRangeException from the per-item read at items[i] =
_list[startIndex + i].

Author: claude

Runtime/Observable/ObservableList.cs

@@ -168,6 +168,7 @@ public void RemoveAt(int index)
 
         public void RemoveRange(int startIndex, int count)
         {
+            if (startIndex < 0) throw new ArgumentOutOfRangeException(nameof(startIndex));
             if (count < 0) throw new ArgumentOutOfRangeException(nameof(count));
 
             lock (SyncRoot)