Combobox: text input + floating listbox with consumer-driven filtering,
allowCustomValue, and aria-autocomplete support.
DropdownMenu: activation-mode menu with menuitem, menuitemcheckbox,
menuitemradio roles, keyboard navigation, and typeahead.
ContextMenu: right-click triggered menu with virtual anchor positioning
at pointer coordinates.
Toast: imperative API (toast(), toast.success/error/dismiss/clear) with
reactive signal-based store and Toast.Region declarative renderer.
