`ObservableStack<T>.CreateView` reverses the order of a non-empty source, then desyncs on `Pop`

Summary

When CreateView is called on an ObservableStack<T> that already contains elements, the view is built in the reverse order of the source.

Creating the view on an empty stack and populating it afterwards works correctly, which is why the existing tests do not catch this.

Repro

var stack = new ObservableStack<int>(new[] { 1, 2, 3 });
var view = stack.CreateView(x => x);
var notify = stack.ToNotifyCollectionChanged();

var removed1 = -1;
stack.CollectionChanged += (in args) => { removed1 = args.OldItem; };

var removed2 = -1;
notify.CollectionChanged += (sender, args) => { removed2 = (int)args.OldItems[args.OldStartingIndex]; };

stack.ToArray();                   // 3, 2, 1
view.ToArray();                    // 1, 2, 3   <-- reversed

var popped = stack.Pop();          // returns 3
stack.ToArray();                   // 2, 1
view.ToArray();                    // 2, 3      <-- 1 was removed instead of 3

Assert.Equal(3, popped);           // pass
Assert.Equal(3, removed1);         // pass
Assert.Equal(3, removed2);         // fail. removed2 is actually 1

Expected:

view enumerates in the same order as stack (top to bottom), and the two stay in sync across Push/`P

Start from ObservableStack.CreateView and the existing tests for creating a view, then reproduce the non-empty-stack case from the issue with ToArray, Pop, and collection-change notifications. Add regression coverage showing that view enumeration matches the stack and that Pop removes the same item from both; run the ObservableStack test suite to verify Push and Pop synchronization.