feat: add IBL demo with normal mapping and procedural environment lighting

Fix pbr_demo, multi_light_demo, and shadow_demo to use the new 7-param
create_pbr_pipeline with PBR texture bind group (4-entry: albedo+normal)
and IBL bind group. Create ibl_demo showcasing a 7x7 metallic/roughness
sphere grid with IBL-based ambient lighting via BRDF LUT integration.

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

Cargo.toml

@@ -14,6 +14,7 @@ members = [
     "examples/pbr_demo",
     "examples/multi_light_demo",
     "examples/shadow_demo",
+    "examples/ibl_demo",
 ]
 
 [workspace.dependencies]