Phase 4: add container-nav fragment handler for market links

Market app serves marketplace links as a fragment at
/internal/fragments/container-nav for consumption by blog and events.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

templates/fragments/container_nav_markets.html

@@ -0,0 +1,9 @@
+{# Market links nav — served as fragment from market app #}
+{% for m in markets %}
+  <a
+    href="{{ market_url('/' + post_slug + '/' + m.slug + '/') }}"
+    class="{{styles.nav_button_less_pad}}">
+    <i class="fa fa-shopping-bag" aria-hidden="true"></i>
+    <div>{{m.name}}</div>
+  </a>
+{% endfor %}