Add dynamic menu block based on site structure hierarchy

Implements SiteStructureMenuBlock that renders a navigation menu
from the ancestor entity (term, user, or group) through all
content_page hierarchies. Features configurable depth, active
trail highlighting, and proper cache invalidation.

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

site_structure.module

@@ -191,6 +191,29 @@ function _site_structure_creates_circular_reference(int|string $node_id, int|str
   return FALSE;
 }
 
+/**
+ * Implements hook_theme().
+ */
+function site_structure_theme(): array {
+  return [
+    'site_structure_menu' => [
+      'variables' => [
+        'ancestor' => NULL,
+        'tree' => [],
+        'active_trail' => [],
+        'show_ancestor_title' => TRUE,
+      ],
+    ],
+    'site_structure_menu_tree' => [
+      'variables' => [
+        'items' => [],
+        'active_trail' => [],
+        'depth' => 0,
+      ],
+    ],
+  ];
+}
+
 /**
  * Implements hook_token_info().
  */