Add Worlds page and nav link; first world: Delph Woods

- worlds.html: new page matching videos.html style, grid of world cards
  with illustrated preview thumbnails and Explore links
- World card CSS added to style.css (.world-card, .world-card-preview,
  .world-enter)
- Delph Woods card links to worlds/delphwoods.html with a woodland SVG
  preview, description, and location/3DGS chips
- Worlds added to nav and footer Product column on all 5 existing pages
- CTA band on worlds page promotes consultancy for commissioning

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

index.html

@@ -32,6 +32,7 @@
           <a href="opensource.html">Open Source</a>
           <a href="consultancy.html">Consultancy</a>
           <a href="videos.html">Videos</a>
+          <a href="worlds.html">Worlds</a>
         </nav>
         <button id="theme-toggle" class="theme-toggle" aria-label="Toggle dark mode">
           <svg class="icon-moon" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M21 12.79A9 9 0 1111.21 3 7 7 0 0021 12.79z"/></svg>
@@ -318,6 +319,7 @@
             <a href="devkits.html">Devkits</a>
             <a href="opensource.html">Open Source</a>
             <a href="videos.html">Videos</a>
+            <a href="worlds.html">Worlds</a>
             <a href="consultancy.html">Consultancy</a>
           </div>
         </div>