Add model import support to Poly Haven importer

Browser gets Textures/Models tabs (per-type catalog cache). Model file
JSON is parsed into per-set texture maps (generic set-prefix matching),
FBX is downloaded with resolution fallback, one URP/Lit material is
built per set via the shared PbrMaterialBuilder (which now returns the
created Material), and new ModelImportUtil remaps FBX material slots to
the built materials by set-name matching.

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

Assets/Scripts/Editor/WebAssets/ModelImportUtil.cs.meta

@@ -0,0 +1,2 @@
+fileFormatVersion: 2
+guid: a54d5f7a2461bb54ca28df0d87665f8e