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>

examples/ibl_demo/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "ibl_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