SelectPanel, FilteredActionList: typing re-renders every item #8717

Part of #8676 (P1). Measure with #8714 before and after.

Problem

• FilteredActionList.tsx renders each item through MappedActionListItem (:628), which passes an inline onSelect to ActionList.Item (:651).
• SelectPanel.tsx builds an inline onAction for every item (:714).
• None of the ActionList, FilteredActionList, SelectPanel, DataTable or TreeView components uses React.memo.

Measured: with 300 items and a leading visual that counts its renders, typing one character that matches every item re-rendered all 300.

FilteredActionList/useAnnouncements.tsx also runs JSON.stringify over every item (getAnnouncementState, :63) and document.querySelector('live-region') (:82) on every render, even with announcements disabled. That was verified in code but not measured. Draft #8641 replaces the querySelector but keeps the JSON.stringify.

Proposed fix

• Wrap MappedActionListItem and ActionList.Item in React.memo.
• Route onSelect and onAction through a ref so item props stay stable.
• Memoize the announcement state, and skip it when announcements are disabled.
• Consider deferring filtering with useDeferredValue, the way Autocomplete defers its input (AutocompleteDeferredInputContext).

Memoization only pays off when consumers keep items stable, so measure with both stable and inline items. The fix must work on React 18 and 19.

Acceptance criteria

• With stable items, typing re-renders only the items whose props changed. Add a test that counts renders.
• Selection, keyboard navigation, active-descendant and announcement behavior don't change.
• The PR reports before and after numbers from Add repeatable performance benchmarks for list, tree and button stress cases #8714, with stable and inline items.