fix: Hard main template rendering lock

- Added $rendering_main_template flag that blocks all other renders
- Reduced MAX_RENDER_DEPTH from 5 to 2
- template-wrapper.php passes is_main_template=true to enable hard lock
- Any render attempt during main template rendering is blocked

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

includes/Frontend/template-wrapper.php

@@ -60,7 +60,8 @@ get_header();
     if ( $template_name ) {
         try {
             // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped
-            echo $plugin->render( $template_name, $context );
+            // Pass true for is_main_template to set the hard rendering lock.
+            echo $plugin->render( $template_name, $context, true );
         } catch ( \Exception $e ) {
             if ( WP_DEBUG ) {
                 echo '<div class="fedistream-error">';