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>
7x7 grid of spheres demonstrating PBR material variation:
metallic increases along X axis, roughness along Y axis.
Uses dynamic UBO pattern for both camera and material uniforms.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
