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>

templates/site-structure-menu.html.twig

@@ -0,0 +1,63 @@
+{#
+/**
+ * @file
+ * Template for the site structure menu block.
+ *
+ * Available variables:
+ * - ancestor: The ancestor entity (term, user, or group).
+ * - tree: Array of menu tree items, each containing:
+ *   - id: Node ID.
+ *   - title: Node title.
+ *   - url: Node URL.
+ *   - depth: Current depth level.
+ *   - children: Array of child items.
+ *   - entity: The node entity.
+ * - active_trail: Array of node IDs in the active trail.
+ * - show_ancestor_title: Whether to show the ancestor title as header.
+ */
+#}
+{{ attach_library('site_structure/menu') }}
+{% if tree is not empty %}
+<nav class="site-structure-menu" aria-label="{{ 'Site structure navigation'|t }}">
+  {% if show_ancestor_title and ancestor %}
+    <h2 class="site-structure-menu__title">
+      {% if ancestor.toUrl is defined %}
+        <a href="{{ ancestor.toUrl }}">{{ ancestor.label }}</a>
+      {% else %}
+        {{ ancestor.label }}
+      {% endif %}
+    </h2>
+  {% endif %}
+
+  <ul class="site-structure-menu__list site-structure-menu__list--level-1">
+    {% for item in tree %}
+      {{ _self.menu_item(item, active_trail, 1) }}
+    {% endfor %}
+  </ul>
+</nav>
+{% endif %}
+
+{% macro menu_item(item, active_trail, depth) %}
+  {% set is_active = item.id in active_trail %}
+  {% set has_children = item.children is not empty %}
+  {% set classes = [
+    'site-structure-menu__item',
+    'site-structure-menu__item--level-' ~ depth,
+    is_active ? 'site-structure-menu__item--active-trail',
+    has_children ? 'site-structure-menu__item--has-children',
+  ] %}
+
+  <li{{ create_attribute().addClass(classes) }}>
+    <a href="{{ item.url }}" class="site-structure-menu__link{% if is_active %} site-structure-menu__link--active-trail{% endif %}">
+      {{- item.title -}}
+    </a>
+
+    {% if has_children %}
+      <ul class="site-structure-menu__list site-structure-menu__list--level-{{ depth + 1 }}">
+        {% for child in item.children %}
+          {{ _self.menu_item(child, active_trail, depth + 1) }}
+        {% endfor %}
+      </ul>
+    {% endif %}
+  </li>
+{% endmacro %}