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/EditorWebRequest.cs

@@ -0,0 +1,84 @@
+using System;
+using UnityEditor;
+using UnityEngine;
+using UnityEngine.Networking;
+
+namespace Chameleon.Editor.WebAssets
+{
+    /// <summary>
+    /// Non-blocking UnityWebRequest transport for editor tools. All requests poll on
+    /// EditorApplication.update — never blocks the editor. Shared by the Poly Haven
+    /// and ambientCG importers.
+    /// </summary>
+    public static class EditorWebRequest
+    {
+        public const string UserAgent = "Chameleon-Unity-AssetImporter/1.0";
+
+        /// <summary>GET a URL and return the response body as text.</summary>
+        public static void GetText(string url, Action<string> onDone, Action<string> onError)
+        {
+            var req = UnityWebRequest.Get(url);
+            req.SetRequestHeader("User-Agent", UserAgent);
+            Send(req, () => onDone?.Invoke(req.downloadHandler.text), onError);
+        }
+
+        /// <summary>Fetches an image as a Texture2D (browser thumbnails, previews).</summary>
+        public static void GetTexture(string url, Action<Texture2D> onDone, Action<string> onError)
+        {
+            var req = UnityWebRequestTexture.GetTexture(url);
+            req.SetRequestHeader("User-Agent", UserAgent);
+            Send(req, () =>
+            {
+                var tex = DownloadHandlerTexture.GetContent(req);
+                // Survive play-mode enter/exit cleanup — otherwise Unity destroys these
+                // loose runtime textures and browser previews go blank.
+                if (tex)
+                    tex.hideFlags = HideFlags.HideAndDontSave;
+                onDone?.Invoke(tex);
+            }, onError);
+        }
+
+        /// <summary>Streams a file straight to disk (no RAM peak on big downloads).</summary>
+        public static void DownloadFile(string url, string destPath, Action onDone,
+            Action<string> onError, Action<float> onProgress = null)
+        {
+            var req = new UnityWebRequest(url, UnityWebRequest.kHttpVerbGET);
+            req.downloadHandler = new DownloadHandlerFile(destPath) { removeFileOnAbort = true };
+            req.SetRequestHeader("User-Agent", UserAgent);
+            Send(req, () => onDone?.Invoke(), onError, onProgress);
+        }
+
+        public static void Send(UnityWebRequest req, Action onSuccess, Action<string> onError,
+            Action<float> onProgress = null)
+        {
+            var op = req.SendWebRequest();
+            void Tick()
+            {
+                if (onProgress != null && !op.isDone)
+                    onProgress(req.downloadProgress);
+
+                if (!op.isDone)
+                    return;
+
+                EditorApplication.update -= Tick;
+                try
+                {
+                    if (req.result != UnityWebRequest.Result.Success)
+                    {
+                        onError?.Invoke($"{req.error} ({req.url})");
+                    }
+                    else
+                    {
+                        onProgress?.Invoke(1f);
+                        onSuccess?.Invoke();
+                    }
+                }
+                finally
+                {
+                    req.Dispose();
+                }
+            }
+            EditorApplication.update += Tick;
+        }
+    }
+}