A Dictionary<string,UiElement> registry on UiRoot with RegisterWindow +
Show/Hide/Toggle. Show/Hide flip UiElement.Visible (already gates
Draw/Tick/HitTest); Toggle returns the new visibility; unknown names are
no-ops. WindowNames.Inventory const shared by mount/registry/toggle.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
