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/Admin/views/settings.php

@@ -48,7 +48,7 @@ $stats = WP_Recache_Admin::get_cache_stats();
                     <tr>
                         <th scope="row">Exclude URLs</th>
                         <td>
-                            <textarea name="wp_recache_exclude_urls[]" rows="5" cols="50" class="large-text"><?php echo esc_textarea(implode("\n", get_option('wp_recache_exclude_urls', ['/cart', '/checkout', '/my-account']))); ?></textarea>
+                            <textarea name="wp_recache_exclude_urls" rows="5" cols="50" class="large-text"><?php echo esc_textarea(implode("\n", get_option('wp_recache_exclude_urls', ['/cart', '/checkout', '/my-account']))); ?></textarea>
                             <p class="description">One URL per line. These pages will not be cached.</p>
                         </td>
                     </tr>
@@ -131,23 +131,3 @@ $stats = WP_Recache_Admin::get_cache_stats();
         <?php submit_button('Save Settings'); ?>
     </form>
 </div>
-
-<script>
-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);
-        }
-    });
-}
-</script>