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.lock

@@ -717,6 +717,21 @@ dependencies = [
  "winit",
 ]
 
+[[package]]
+name = "ibl_demo"
+version = "0.1.0"
+dependencies = [
+ "bytemuck",
+ "env_logger",
+ "log",
+ "pollster",
+ "voltex_math",
+ "voltex_platform",
+ "voltex_renderer",
+ "wgpu",
+ "winit",
+]
+
 [[package]]
 name = "indexmap"
 version = "2.13.0"