feat: add PBR demo with metallic/roughness sphere grid

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>

examples/pbr_demo/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "pbr_demo"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+voltex_math.workspace = true
+voltex_platform.workspace = true
+voltex_renderer.workspace = true
+wgpu.workspace = true
+winit.workspace = true
+bytemuck.workspace = true
+pollster.workspace = true
+env_logger.workspace = true
+log.workspace = true