Add TDD regression test for issue #11039: verify that
ActorSelectionOverlay does not define its own _render() method.
The method was renamed to _refresh_display() to prevent shadowing
the Textual Widget._render method which caused NoneType crashes
during layout engine calls.
The `ActorSelectionOverlay` class inherits from `textual.widgets.Static`,
which already defines a `_render()` method used for rendering widget content.
Our private `_render()` was shadowing that method, causing incorrect repaint
behavior and interfering with Textual's layout pass. This renames the internal
method to `_refresh_display()` while keeping the public API unchanged.\n\nISSUES CLOSED: #11042