Purge page caches after every write; report it in the result (0.1.18)

[mostafasoufi]

Fixes #11

What changes

• After every write and undo, the plugin asks the active page caches to drop what changed:
  • SEO title, description, canonical, noindex and the opening paragraph: the post's URL, plus the home page when the post is the static front page.
  • Alt text: the post the image belongs to (the change's post_id, else the post it was uploaded to).
  • Redirects: the source URL (and that post's purge when the source is a post).
  • llms.txt and AI crawler rules: /llms.txt or /robots.txt, as before.
• New caches, each through its public API and only when active (all guarded, all wrapped in try/catch, never fatal):

Cache	Call (verified in its source)
WP-Optimize 4.7.0	WPO_Page_Cache::instance()->is_enabled(), then WPO_Page_Cache::delete_cache_by_url( $url ) (false = failed)
Nginx Helper 2.4.1	global $nginx_purger → purge_url( $url, false ), only when $nginx_helper_admin->options['enable_purge']
Breeze 2.6.0	do_action( 'purge_post_cache', $post_id ) (its documented call), posts only
Kinsta	$kinsta_cache->kinsta_cache_purge->initiate_purge( $post_id, 'post' ); purge_complete_caches() for llms/robots
WP Engine	WpeCommon::purge_varnish_cache( $post_id ); purge_varnish_cache() for llms/robots
Pantheon Advanced Page Cache	pantheon_wp_clear_edge_paths( $paths )

Existing: WP Rocket (now rocket_clean_home() for the home page, since purging its folder by URL would empty the whole cache), LiteSpeed, W3 Total Cache, WP Super Cache, SiteGround.

• Kinsta and WP Engine have no one-URL purge, so they purge everything only for the site-wide files; a redirect source that is not a post is left alone there. Kinsta's must-use plugin is not public; its calls are the ones WP Rocket's own Kinsta integration uses. WP Engine's signature was checked in a copy of its must-use plugin.
• Cloudflare (official plugin 4.14.4): still skipped. It has no public function or action to purge a URL. Its purge method lives on an object it does not expose, and its cloudflare_purge_url_actions filter is read when Cloudflare loads, which is before MonoRanks (plugins load alphabetically). The opening-paragraph write saves the post, so Cloudflare's own transition_post_status hook already purges in that case.

Result field

Each successful write result has cache: { purged: string[], failed: string[], none: bool }. All other fields are unchanged; refused changes carry no cache. Documented in docs/api.md.

New filter monoranks_purge_urls and action monoranks_purged_cache; CachePurger::purge_file() keeps its old return value.

Tests

Unit tests (Brain Monkey) for every adapter: active and inactive, failure, throw, home page, front page, redirect-to-post, plus a Writer test for the cache field. No WP-Optimize integration test (optional; it would need the plugin and its advanced-cache setup in the CI WordPress).

Version 0.1.18 (header, constant, Stable tag, CHANGELOG, readme changelog). Not released.