feat(website): add Built-in category tag for stdlib entries

Stdlib entries now show a "Built-in" tag alongside their regular
category tag. The tag is clickable and filters to all 17 stdlib
entries. Added to data-cats attribute so existing filter logic works.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

website/static/style.css

@@ -347,6 +347,12 @@ th[data-sort].sort-asc::after {
   white-space: nowrap;
 }
 
+.tag-source {
+  background: var(--bg-input);
+  color: var(--text-muted);
+  font-weight: 600;
+}
+
 /* === Arrow Column === */
 .col-arrow {
   width: 2.5rem;

website/templates/index.html

@@ -84,7 +84,7 @@
       <tr
         class="row"
         role="button"
-        data-cats="{{ entry.categories | join('||') }}"
+        data-cats="{{ entry.categories | join('||') }}{% if entry.source_type == 'Built-in' %}||Built-in{% endif %}"
         data-groups="{{ entry.groups | join('||') }}"
         tabindex="0"
         aria-expanded="false"
@@ -122,6 +122,11 @@
             {{ cat }}
           </button>
           {% endfor %}
+          {% if entry.source_type == 'Built-in' %}
+          <button class="tag tag-source" data-type="cat" data-value="Built-in">
+            Built-in
+          </button>
+          {% endif %}
           <button class="tag tag-group" data-type="group" data-value="{{ entry.groups[0] }}">
             {{ entry.groups[0] }}
           </button>