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/deferred_lighting.wgsl

@@ -42,6 +42,10 @@ struct ShadowUniform {
     light_view_proj: mat4x4<f32>,
     shadow_map_size: f32,
     shadow_bias: f32,
+    _padding: vec2<f32>,
+    sun_direction: vec3<f32>,
+    turbidity: f32,
+    sh_coefficients: array<vec4<f32>, 7>,
 };
 
 @group(2) @binding(0) var t_shadow: texture_depth_2d;