Add video layer: load VJ video loops that blend with the visuals

Loads a muted looping video as a layer above the WebGL canvas with opacity,
blend mode and fit controls; included in MP4 recordings and droppable.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/output.html

@@ -10,6 +10,9 @@
 <body class="output">
   <canvas id="gl"></canvas>
 
+  <!-- Video layer (VJ loop) that blends with the generated visuals -->
+  <video id="bg-video" class="bg-video" loop muted playsinline></video>
+
   <!-- Image overlay layer -->
   <div id="overlay-images"></div>