Pagefind Search 1.0.0-beta1

Feature-complete for 1.0.0. Beta because the accessibility work has not yet
been verified at runtime and the Setup Wizard has not yet been exercised by
a human outside the test suite.

New in this release
- Setup Wizard: a guided path from install to working search at
  /admin/config/search/pagefind/setup. The dashboard and help text now
  funnel first-run users into it.
- Multisite collections: merge another site's remote Pagefind index into
  React search displays.
- Synonyms: content is automatically re-staged via cron when synonyms
  change, with a dashboard call-to-action.
- Promoted results (best bets) on React search displays, rendered as a
  pinned featured card above organic results.
- Staging performance: synonym groups are memoized and the entity cache is
  reset per Drush chunk.

Hardening
- WCAG 2.1 AA remediation across the React displays, CSS, legacy template
  and admin forms. See ACCESSIBILITY.md for scope and limits.
- Operational truthfulness: honest Drush exit codes, hook_requirements
  health checks, real logging on failure paths.
- Cross-process index build lock, build timeout, and index-removal
  propagation.
- 17 new documentation files for site builders and developers.

Known limitations
- Accessibility conformance is a build target, not an audited claim. No
  browser, screen-reader or automated axe pass has been run. Do not treat
  ACCESSIBILITY.md as a VPAT or Section 508 conformance statement.
- The legacy web-component search surface does not support promoted
  results or multisite collections; both are React-display only.
- The Setup Wizard is new in this release and has limited real-world use.

Verified on 8b6ce38: drupal.org pipeline 904898 green across all jobs
(composer, composer-lint, phpcs, phpstan, stylelint, eslint, cspell,
phpunit). Locally: Kernel 128 tests / 1450 assertions, Functional 39 tests
/ 429 assertions, PHPStan clean, PHPCS clean. Bundle verified rebuilt
byte-identical from pagefind-react-ui 1.0.0-beta1 (af317fc).