feat: add multi-light demo with orbiting point lights and spot light

Fix pbr_demo to use LightsUniform/LightData instead of old LightUniform.
Create multi_light_demo with 5 PBR spheres (varying metallic), a ground
plane, 4 colored orbiting point lights, a directional fill light, and a
spot light from above.

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

Cargo.toml

@@ -12,6 +12,7 @@ members = [
     "examples/hierarchy_demo",
     "examples/asset_demo",
     "examples/pbr_demo",
+    "examples/multi_light_demo",
 ]
 
 [workspace.dependencies]