Self-referencing property access inside a hook accesses the backing
store directly without calling the corresponding hook. Previously
such usages were skipped entirely, which caused false positives for
memoizing patterns like `$this->id ??= ...`.

Now these usages are recorded with a non-propagating flag: the
property is marked as used, but the graph walk does not continue
into the hook body. This correctly models the difference between
accessing the property (backing store) and calling a hook.