Add height fog system (高度雾方案)

[Copilot]

Implements an exponential height fog effect as a full-screen post-process pass inserted between the forward pass and tone-mapping.

Fog Algorithm

Analytically integrates exponential density along the view ray:

density(h) = FogDensity * exp(-HeightFalloff * max(h - BaseHeight, 0))

Closed-form solution handles both horizontal and angled rays correctly. World position is reconstructed from depth buffer using InvViewProj.

New Components

• HeightFogFeatureProcessor — manages fog params UBO, exports HeightFogParamsBuffer to render graph each frame; density defaults to 0 (no-op until component attached)
• HeightFogPass — FullScreenPass reading FWD_CL + FWD_DS → HeightFogOutput
• HeightFogComponent — serializable world component exposing: fog color, inscatter color, density, height falloff, base/max height, start distance; resets density to 0 on detach

Pipeline Changes

• DefaultForwardPipeline::Collect conditionally inserts HeightFogPass when HeightFogFeatureProcessor is present, redirecting PostProcessingPass input via new SetColorInput()
• HeightFogFeature::Init() registered alongside other features in RenderModule

Cross-API Depth Reconstruction

• Added invViewProject (= viewProject.Inverse()) to SceneViewInfo / ViewInfo HLSL struct
• HeightFogParams::clipYSign corrects NDC Y-axis convention: +1 for Vulkan (Y=-1 at top), -1 for DX12/Metal (Y=+1 at top), set automatically from RHI::GetDevice()->GetConstants().flipY

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