Query Filter: Add focus-only keyboard type-ahead

[jamesbregenzer]

Thanks @StevenDufresne for opening #541 and for the clear native-select behavior to aim for.

Summary

This adds focus-only keyboard type-ahead to the open Query Filter using its existing Interactivity API. Typed label prefixes move focus among enabled options without filtering the list or changing checked state. A short 500 ms prefix window supports multiple characters; repeating a single starting letter cycles through matching options. A miss leaves focus unchanged, and a failed prefix extension starts a fresh search from its latest character.

Escape and the existing Tab focus trap are preserved. Space continues to activate checkboxes and radios natively, and radio arrow-key behavior is unchanged. Modifier shortcuts, IME composition, and editable controls are left alone. This patch does not change Apply/Clear or submission behavior; PR #748's empty-form submission fix remains separate.

Verification

• Query Filter JavaScript lint, Prettier, and the production build passed.
• The DOM harness passed red/green acceptance cases and killed a mutation that removed the focus call.
• Chromium checks passed against WordPress's Interactivity API runtime and a page rendered by the PHP block renderer. An independent rerun passed all 12 PHP-rendered checks with no browser errors.

These checks used local fixtures, not the live production site. The patch was checked against trunk at 3b104f0554e80d7712d90d92e6ac3c46a3716c41 on 2026-10-15.