Add in-house comments (Lib\Comments)

A reusable comment thread any sidecar can attach to any page, tied to
real logged-in accounts (never anonymous), auto-approved on submission
with hide/delete moderation at /admin/comments — only a verified account
can post, so there's no anonymous-spam vector to pre-vet against.

Framework-level (novaconium/lib, novaconium/migrations, novaconium/pages),
not App/migrations, matching Admin auth and Media manager's shape. A page
needs its own sidecar to use it — this repo has no client-side JS, so
comments ride the same server-rendered POST pattern as every other
dynamic feature, which is also what excludes a page from the static
cache. App/pages/blog/comments-demo/ demonstrates the pattern without
touching existing posts that are referenced elsewhere as the
sidecar-less example.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

App/pages/blog/_layout/layout.twig

@@ -17,6 +17,13 @@
         </aside>
         <article>
             {% block blog_content %}{% endblock %}
+            {# Only pages whose sidecar opts in by returning a 'comments'
+               key get a thread — see /admin/docs/comments and
+               App/pages/blog/hello-world/index.php. A sidecar-less post
+               never has this key, so it's silently skipped. #}
+            {% if comments is defined %}
+                {% include '_partials/comments/thread.twig' %}
+            {% endif %}
         </article>
     </div>
 {% endblock %}