feat(renderer): add normal mapping and procedural IBL to PBR shader

- Add tangent input (location 3) and TBN computation in vertex shader
- Add normal map sampling (group 1, bindings 2-3) for tangent-space normal mapping
- Add BRDF LUT binding (group 4, bindings 0-1) for specular IBL
- Add procedural sky environment function for diffuse/specular IBL
- Replace flat ambient with split-sum IBL approximation
- Add pbr_texture_bind_group_layout (4 entries: albedo + normal)
- Add create_pbr_texture_bind_group helper and flat_normal_1x1 texture
- Update create_pbr_pipeline to accept ibl_layout parameter (group 4)

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

crates/voltex_renderer/src/lib.rs

@@ -18,7 +18,7 @@ pub use gpu::{GpuContext, DEPTH_FORMAT};
 pub use light::{CameraUniform, LightUniform, LightData, LightsUniform, MAX_LIGHTS, LIGHT_DIRECTIONAL, LIGHT_POINT, LIGHT_SPOT};
 pub use mesh::Mesh;
 pub use camera::{Camera, FpsController};
-pub use texture::GpuTexture;
+pub use texture::{GpuTexture, pbr_texture_bind_group_layout, create_pbr_texture_bind_group};
 pub use material::MaterialUniform;
 pub use sphere::generate_sphere;
 pub use pbr_pipeline::create_pbr_pipeline;