Pull-based contract for a select-shell host so a projected
<cngx-select-search> reads and writes the search term and
forwards navigation keys into the listbox AD without ancestor
injection of the concrete shell class.
searchTerm - ModelSignal<string>. Empty disables the filter.
listboxRef - read-only signal to the listbox; used by the
search element to forward ArrowUp/Down, Enter, Home,
End, Escape while focus stays in the input.
close() - closes panel and returns focus to the trigger
(Escape path).