Add Poly Haven importer editor tool

Tools/Poly Haven Importer: fetches asset metadata/files via the public
Poly Haven API, browsable texture catalog with searchable thumbnail
grid, per-map resolution selection, async non-blocking downloads, and
automatic URP/Lit material build (roughness->smoothness repacking into
_MetallicGlossMap, ARM unpacking as fallback for missing AO/Rough/Metal
maps, linear import settings for data textures). Thumbnails survive
play mode via HideAndDontSave + destroyed-cache recovery.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Assets/Scripts/Editor/PolyHaven.meta

@@ -0,0 +1,8 @@
+fileFormatVersion: 2
+guid: e7186f7ca5e56f14382071f77388acaa
+folderAsset: yes
+DefaultImporter:
+  externalObjects: {}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: