New Tools/ambientCG Importer window (catalog browser, ZIP download with
selective map extraction, URP/Lit material build). Generic pieces moved
from the PolyHaven tool into Editor/WebAssets (EditorWebRequest,
PbrMapType, PbrMaterialBuilder, ImportUtil, AssetBrowserGrid); PolyHaven
API/window now delegate to the shared code, behavior unchanged.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
