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

includes/Deactivator.php

@@ -14,20 +14,38 @@ class WP_Recache_Deactivator {
      */
     public static function deactivate() {
         self::remove_advanced_cache();
+        self::remove_cache_server();
+        WP_Recache_Activator::set_wp_cache_constant(false);
         self::flush_rewrite_rules();
     }
 
     /**
-     * Remove advanced-cache.php drop-in.
+     * Remove advanced-cache.php drop-in, but only if it is ours.
      */
     private static function remove_advanced_cache() {
         $advanced_cache = WP_CONTENT_DIR . '/advanced-cache.php';
 
-        if (file_exists($advanced_cache)) {
+        if (!file_exists($advanced_cache)) {
+            return;
+        }
+
+        $content = file_get_contents($advanced_cache);
+        if (strpos($content, 'WP Recache') !== false) {
             unlink($advanced_cache);
         }
     }
 
+    /**
+     * Remove the standalone cache server.
+     */
+    private static function remove_cache_server() {
+        $server = WP_CONTENT_DIR . '/wp-recache-cache.php';
+
+        if (file_exists($server)) {
+            unlink($server);
+        }
+    }
+
     /**
      * Flush rewrite rules.
      */