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/PbrMaterialBuilder.cs

@@ -19,7 +19,8 @@ namespace Chameleon.Editor.WebAssets
             public string AssetPath; // "Assets/Poly Haven/.../foo_diffuse.jpg"
         }
 
-        public static void Build(string folder, string displayName, string slug, int maxSize,
+        /// <summary>Builds the material and returns it (null if it could not be created).</summary>
+        public static Material Build(string folder, string displayName, string slug, int maxSize,
             List<Downloaded> maps)
         {
             var byType = new Dictionary<PbrMapType, string>();
@@ -61,7 +62,7 @@ namespace Chameleon.Editor.WebAssets
             if (!shader)
             {
                 Debug.LogError("[PBR Import] URP/Lit shader not found — is URP installed?");
-                return;
+                return null;
             }
 
             var mat = new Material(shader) { name = displayName };
@@ -105,6 +106,7 @@ namespace Chameleon.Editor.WebAssets
             EditorGUIUtility.PingObject(mat);
             Selection.activeObject = mat;
             Debug.Log($"[PBR Import] Material created: {matPath}");
+            return mat;
         }
 
         private static Texture2D Load(string path) => AssetDatabase.LoadAssetAtPath<Texture2D>(path);