Substrate Unlit editor view mode renders black with Blendable GBuffer

When using Substrate materials with SUBSTRATE_GBUFFER_FORMAT==0 (Blendable GBuffer), the Unlit editor view mode renders completely black. TopLayerData.UnlitViewBaseColor is never populated in the blendable path - it stays zero-initialized while the non-blendable path (Format==1) correctly sets it via PackSubstrateOut(). Fix: Assign Export.BaseColor to TopLayerData.UnlitViewBaseColor after SubstrateMaterialExportOut() in the Format==0 path, guarded by USE_DEVELOPMENT_SHADERS.

1. Set project to use Substrate Blendable GBuffer.
2. Make a material, using a substrate slab BSDF.
3. Change any viewport to unlit view mode with that material in it.
4. Notice that the material appears black.