fix: address Phase 1 code review findings

- Serve cache via template_redirect so feeds/404/REST are excluded and
  query conditionals work
- Bypass cache on query strings; key files by md5(host+path) to avoid
  collisions and support multisite
- Write wp-content/wp-recache-cache.php drop-in server and set WP_CACHE
  in wp-config.php so advanced-cache.php actually runs
- Only remove advanced-cache.php on deactivation when it is ours
- Respect DONOTCACHEPAGE, non-200 responses; bots read but never warm
  cache
- Fix comment purge (comment_post_ID) and heartbeat purge ($data arg)
- Fix exclude URLs textarea parsing; purge all when exclusions change
- Enqueue admin purge script on every admin-bar page, not just settings
- Sync tests, add TESTING.md manual test guide

wp-recache.php

@@ -34,11 +34,13 @@ function wp_recache_init() {
         $admin->init();
     }
 
-    if (WP_RECACHE_CACHE_ENABLED) {
+    if (wp_recache_is_cache_enabled()) {
         require_once WP_RECACHE_INC_PATH . 'Cache/WPCache.php';
         $cache = new WP_Recache_WPCache();
         $cache->init();
     }
+
+    add_action('wp_enqueue_scripts', 'wp_recache_enqueue_admin_bar_script');
 }
 
 /**
@@ -54,6 +56,7 @@ register_activation_hook(__FILE__, 'wp_recache_activate');
  * Plugin deactivation.
  */
 function wp_recache_deactivate() {
+    require_once WP_RECACHE_INC_PATH . 'Activator.php';
     require_once WP_RECACHE_INC_PATH . 'Deactivator.php';
     WP_Recache_Deactivator::deactivate();
 }