feat: add 404 page and tag listing generation

Detect content/_404.md as special page (underscore
prefix convention), store in SiteManifest.page_404,
render to 404.html in output root.

Add collect_tags() grouping all tagged content into
BTreeMap for deterministic output. render_tag_page()
renders through tags/default.html template.

Add 5 tests. Test suite: 78 → 83, all passing.

docs/templates/tags/default.html

@@ -0,0 +1,15 @@
+{% extends "base.html" %} {% block content %}
+<article class="section-index">
+    <h1>Tagged: {{ tag }}</h1>
+    <nav class="section-nav">
+        {% for item in items %}
+        <a href="{{ prefix }}{{ item.path }}" class="section-link">
+            <strong>{{ item.frontmatter.title }}</strong>
+            {% if item.frontmatter.description %}
+            <span>{{ item.frontmatter.description }}</span>
+            {% endif %}
+        </a>
+        {% endfor %}
+    </nav>
+</article>
+{% endblock content %}