Add ambientCG importer and extract shared web-asset import framework

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>

Assets/Scripts/Editor/WebAssets/ImportUtil.cs

@@ -0,0 +1,66 @@
+using System.IO;
+using UnityEditor;
+
+namespace Chameleon.Editor.WebAssets
+{
+    /// <summary>Small shared helpers for the web-asset importer windows.</summary>
+    public static class ImportUtil
+    {
+        public static string SanitizeFileName(string name)
+        {
+            foreach (var c in Path.GetInvalidFileNameChars())
+                name = name.Replace(c, ' ');
+            return name.Trim();
+        }
+
+        /// <summary>"2k"/"2K" -> 2. Unknown strings sort last.</summary>
+        public static int ResolutionRank(string res)
+        {
+            if (string.IsNullOrEmpty(res)) return int.MaxValue;
+            var digits = res.TrimEnd('k', 'K');
+            return int.TryParse(digits, out var n) ? n : int.MaxValue;
+        }
+
+        /// <summary>Maps a "2k"-style resolution to a valid Unity max texture size.</summary>
+        public static int ResolutionToMaxSize(string res)
+        {
+            int k = ResolutionRank(res);
+            int px = k * 1024;
+            int[] valid = { 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384 };
+            foreach (var v in valid)
+                if (px <= v) return v;
+            return 16384;
+        }
+
+        public static void EnsureFolder(string folder)
+        {
+            if (AssetDatabase.IsValidFolder(folder))
+                return;
+            var parts = folder.Split('/');
+            string cur = parts[0]; // "Assets"
+            for (int i = 1; i < parts.Length; i++)
+            {
+                string next = $"{cur}/{parts[i]}";
+                if (!AssetDatabase.IsValidFolder(next))
+                    AssetDatabase.CreateFolder(cur, parts[i]);
+                cur = next;
+            }
+        }
+
+        /// <summary>
+        /// Non-destructive folder-collision handling: if the folder exists, asks the user
+        /// (New folder / Cancel / Overwrite). Returns the folder to use, or null on cancel.
+        /// </summary>
+        public static string ResolveFolderCollision(string folder, string dialogTitle)
+        {
+            if (!AssetDatabase.IsValidFolder(folder))
+                return folder;
+
+            int choice = EditorUtility.DisplayDialogComplex(dialogTitle,
+                $"Folder already exists:\n{folder}", "New folder", "Cancel", "Overwrite");
+            if (choice == 1) return null;                          // Cancel
+            if (choice == 0) return AssetDatabase.GenerateUniqueAssetPath(folder); // New folder
+            return folder;                                          // Overwrite
+        }
+    }
+}