fix(core): announce Selector search result counts to screen readers

[bhamodi]

Summary

In hasSearch mode, typing into the Selector popover's search input filters the option list silently. The filteredItems memo recomputes on every keystroke (Selector.tsx:640-648) and renderOptions swaps in a visual "No results found" empty state (Selector.tsx:911), but nothing was announced to assistive technology -- the file never imported useAnnounce. A screen-reader user narrowing a long list heard only their own typing, with no signal for how many options remained or that the list had emptied.

Typeahead already gives this feedback: BaseTypeahead announces result counts as you type via the repo's useAnnounce hook (BaseTypeahead.tsx:431-438). This brings Selector to parity, reusing the exact same strings -- `${n} result${n === 1 ? '' : 's'}` and "No results found" -- routed through the same hook and its persistently-mounted polite live region.

An effect keys off the existing filteredItems memo and the search state (Selector.tsx:691-706), so it covers every way the filtered list changes through one code path and adds no new prop or render branch. It announces only while the popover is open with a non-empty query; when the query is emptied or the popover closes it clears the region so stale status text does not linger.

Changes

• Selector/Selector.tsx: import useAnnounce; add an effect that announces "3 results" / "1 result" / "No results found" politely while searching (matching BaseTypeahead's exact wording), and clears the region when the search context goes away.

Test plan

• node_modules/.bin/vitest run --root . packages/core/src/Selector -- 49 pass (Selector.test.tsx). Four new tests under hasSearch > result announcements:
  • announces the match count politely while searching ("2 results")
  • announces the singular form when one option matches (asserted against /^1 result$/ so it cannot pass on a plural "1 results")
  • announces "No results found" when nothing matches
  • does not announce results until the user searches (closed popover / empty query)
  • The three positive-announcement tests were confirmed failing before the fix; the "does not announce" test guards against announcing on open or when idle.
• node_modules/.bin/tsc --project packages/core/tsconfig.json --noEmit -- clean.
• node_modules/.bin/eslint on changed files -- clean.

Notes

Found during a broader a11y audit of core components; scoped to one fix per PR (does not add the separate Home/End key-handling finding or touch other Selector behavior). Selector.doc.mjs was left unchanged: no new props were added and it has no accessibility/features section, matching how the analogous Typeahead announce behavior is not documented there either.

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