feat(render): Screen-space decal system decoupled from forward/deferred pipelines

[Copilot]

Adds a screen-space decal system designed to be pipeline-agnostic — it works after any opaque pass (forward, deferred, MSAA) by reading only a depth buffer and a color render target.

Architecture

render/core — pipeline-agnostic layer

• DecalFeatureProcessor — manages up to 16 DecalInstance objects; uploads a DecalPassInfo UBO (DECAL_PASS_INFO) to the render graph each frame via Render()
• DecalInstance — stores per-decal OBB (world-to-local matrix), RGBA tint color, and blend factor

render/adaptor — pipeline-specific layer

• DecalPass (extends FullScreenPass) — color and depth resource names are injected at construction, so the pass can be wired to any opaque pass's outputs without modification
• DecalComponent — actor component; creates/destroys DecalInstance via the feature processor, updates the OBB matrix on transform change

Shader (assets/shaders/decal.hlsl)

Full-screen triangle → sample depth → reconstruct world position via InvViewProj → per-decal OBB test (unit-cube in local space) → alpha-blend with painter's algorithm:

float4 localPos = mul(Decals[i].WorldToDecal, float4(worldPos, 1.0));
if (all(abs(localPos.xyz) <= 0.5)) {
    float blend = Decals[i].DecalColor.a * Decals[i].Params.x;
    outColor = lerp(outColor, float4(Decals[i].DecalColor.rgb, 1.0), blend);
}

Pipeline integration (DefaultForwardPipeline)

DecalPass is inserted between the forward pass and post-processing. It uses LoadOp::LOAD + alpha blending to composite onto ForwardColor in-place — no extra buffer, no change to post-processing. When no DecalFeatureProcessor is registered, DecalPass::Setup() is a no-op.

Supporting changes

• SceneViewInfo / HLSL ViewInfo extended with InvViewProj (precomputed in SceneView::Update()) — useful for any depth-reconstruction effect beyond decals
• decal.tech: alpha blending, no depth write, no face culling
• DecalFeatureProcessor and DecalComponent registered in RenderModule

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