feat: volumetric fog and atmospheric haze

[Hekbas]

Implement volumetric fog/haze rendering for atmospheric depth and light scattering effects.

Tasks

• Implement a froxel-based volumetric fog system (3D texture frustum voxelization)
• Compute light scattering through fog volumes (directional + point light contribution)
• Apply fog as a post-geometry composite pass (ray-march or lookup)
• Support distance-based and height-based fog falloff
• Add fog density, color, and scattering parameters to a volume component or global setting
• Editor UI for fog parameters in the Render panel

[Hekbas]

Shipped as volumetric-fog v3.0.3 → volumetric-validation v3.0.4 → volumetric-fog-polish v3.0.5+v3.0.6. Wronski voxel fog with HG phase, IBL multi-scatter, temporal accumulation, FogVolume components, debug viz. Per-froxel culling deferred to #134; world-space sun-shield LUT deferred to #133.