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

assets/admin.js

@@ -0,0 +1,23 @@
+/**
+ * WP Recache admin-bar / settings actions.
+ *
+ * @package WP_Recache
+ */
+
+function wpRecachePurgeAll() {
+    if (!confirm('Are you sure you want to purge all cache?')) {
+        return;
+    }
+
+    jQuery.post(wpRecache.ajaxUrl, {
+        action: 'wp_recache_purge_all',
+        nonce: wpRecache.nonce
+    }, function (response) {
+        if (response.success) {
+            alert('Cache purged successfully');
+            location.reload();
+        } else {
+            alert('Error: ' + response.data);
+        }
+    });
+}