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

@@ -0,0 +1,63 @@
+using System.Collections.Generic;
+using UnityEditor;
+using UnityEngine;
+
+namespace Chameleon.Editor.WebAssets
+{
+    /// <summary>Shared helpers for imported 3D models (FBX etc.).</summary>
+    public static class ModelImportUtil
+    {
+        /// <summary>
+        /// Remaps every source material of an imported model to the given built materials.
+        /// With a single set, all slots get that material; with multiple sets, the source
+        /// material name is matched against the set names (substring, case-insensitive).
+        /// </summary>
+        public static void RemapMaterials(string modelAssetPath, List<(string setName, Material mat)> sets)
+        {
+            if (sets == null || sets.Count == 0)
+                return;
+            var importer = AssetImporter.GetAtPath(modelAssetPath) as ModelImporter;
+            if (!importer)
+            {
+                Debug.LogWarning($"[PBR Import] No model importer at {modelAssetPath} — skipping material remap.");
+                return;
+            }
+
+            // Source material names = the embedded materials Unity generated on import.
+            var srcNames = new List<string>();
+            foreach (var obj in AssetDatabase.LoadAllAssetsAtPath(modelAssetPath))
+                if (obj is Material m)
+                    srcNames.Add(m.name);
+
+            if (srcNames.Count == 0)
+            {
+                Debug.LogWarning($"[PBR Import] {modelAssetPath} exposes no materials to remap.");
+                return;
+            }
+
+            foreach (var name in srcNames)
+            {
+                var target = Pick(name, sets);
+                if (!target)
+                    continue;
+                importer.AddRemap(new AssetImporter.SourceAssetIdentifier(typeof(Material), name), target);
+            }
+            importer.SaveAndReimport();
+            Debug.Log($"[PBR Import] Remapped {srcNames.Count} material slot(s) on {modelAssetPath}.");
+        }
+
+        private static Material Pick(string srcName, List<(string setName, Material mat)> sets)
+        {
+            if (sets.Count == 1)
+                return sets[0].mat;
+
+            string s = srcName.ToLowerInvariant();
+            foreach (var (setName, mat) in sets)
+                if (!string.IsNullOrEmpty(setName) && s.Contains(setName.ToLowerInvariant()))
+                    return mat;
+
+            Debug.LogWarning($"[PBR Import] Material '{srcName}' matched no texture set — using '{sets[0].setName}'.");
+            return sets[0].mat;
+        }
+    }
+}