Tryst::UI::OverlayValidator

@api private

The opposite direction from WidgetDSL#overlay's own guard: a node carrying an overlay placement (#overlay_anchor) whose actual parent isn't a :canvas at all - only reachable via direct Node/Document manipulation, since WidgetDSL#overlay already refuses to run outside a ui.canvas block. Mirrors GridValidator.check_stray_cell exactly; overlay intent can land on any node type (whatever #overlay's block happens to build), so like that check - and unlike a type-dispatched WidgetValidators entry - this can't be keyed off a single node type. Validator calls this directly for every node in the same single tree walk.