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>

Cargo.toml

@@ -11,6 +11,7 @@ members = [
     "examples/many_cubes",
     "examples/hierarchy_demo",
     "examples/asset_demo",
+    "examples/pbr_demo",
 ]
 
 [workspace.dependencies]