Clear the frame to the apron colour

That band at the bottom was not a background mismatch — the canvas was
transparent there. render() never cleared: the stands cover the top and the
out-area quad covers the middle, so it looked complete, but the quad only
reached z=30 and everything below where that projects was see-through. The
page showed through it, which is why it read as a slightly duller green.

The frame is now cleared to pal.out before anything draws, oversized by 8px so
a camera shake cannot expose an edge, and the out-area quad runs to z=60 so the
perspective floor reaches the bottom on any aspect. Verified by sampling alpha
down the bottom third: opaque everywhere, one uniform colour, matching the page
gradient behind it.

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

index.html

@@ -80,6 +80,6 @@
     <p class="start">TAP TO START</p>
   </div>
 </div>
-<script type="module" src="main.js?v=48"></script>
+<script type="module" src="main.js?v=49"></script>
 </body>
 </html>