Use one item counter for Combobox, Menu, and Command #962

Combobox, Menu, ContextMenu, and Command each need to know how many items match the search. They use the count to show the empty state and to announce "N results" for screen readers.

Base UI's own count (useFilteredItems) only works when the root gets an items prop. Our components also filter their own children, so they need their own count.

Today there are two ways to do this:

• feat: [menu, context-menu] add checkbox, radio, and link items #953 and feat: [command] use base ui empty and status #960 add packages/react/shared/item-count/. Each item adds itself to the count while it is shown. Menu, ContextMenu, and Command use it.
• feat: [combobox] use base ui chips, clear, empty, and status #954 adds a separate count on the Combobox root.

Both do the same job.

Proposal

After #953, #954, and #960 merge, move Combobox to shared/item-count and delete its own count. Keep one code path for the empty state and the status message in all four components.

Select's search mode (#958) only shows Select.Empty when items is given. It could use the same helper to work without items too.