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

uninstall.php

@@ -49,3 +49,9 @@ if (file_exists($advanced_cache)) {
         unlink($advanced_cache);
     }
 }
+
+// Remove the standalone cache server.
+$cache_server = WP_CONTENT_DIR . '/wp-recache-cache.php';
+if (file_exists($cache_server)) {
+    unlink($cache_server);
+}