feat(renderer): improve IBL with Hosek-Wilkie sky, SH irradiance, GPU BRDF LUT

- Hosek-Wilkie inspired procedural sky (Rayleigh/Mie scattering, sun disk)
- L2 Spherical Harmonics irradiance (9 coefficients, CPU computation)
- SH evaluation in shader replaces sample_environment for diffuse IBL
- GPU compute BRDF LUT (Rg16Float, higher precision than CPU Rgba8Unorm)
- SkyParams (sun_direction, turbidity) in ShadowUniform

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

crates/voltex_renderer/src/shadow.rs

@@ -144,6 +144,11 @@ pub struct ShadowUniform {
     pub shadow_map_size: f32,
     pub shadow_bias: f32,
     pub _padding: [f32; 2],
+    // Sky parameters (Hosek-Wilkie inspired)
+    pub sun_direction: [f32; 3],
+    pub turbidity: f32,
+    // L2 Spherical Harmonics coefficients: 9 RGB coefficients packed into 7 vec4s (28 floats)
+    pub sh_coefficients: [[f32; 4]; 7],
 }
 
 #[repr(C)]