Fix AI category heading suppressed by project titles containing category name

The heading check used a substring match on the full accumulated categories
string, so a project title like "AI driven interface for SlicerTMS" (in the
IGT and Training section) would prevent the "### AI" heading from ever being
generated. Fix by checking for the full heading string "### <category>" instead.

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

Author: sjh26

_includes/project_generate_category.md

@@ -46,7 +46,8 @@
   {% if page.project_categories contains pw_page_category %}
 
     {% comment %}If if applies, add catergory header.{% endcomment %}
-    {% unless categories contains pw_page_category %}
+    {% assign category_heading = "### " | append: pw_page_category %}
+    {% unless categories contains category_heading %}
 {% capture categories %}
 {{ categories }}
 ### {{ pw_page_category }}