Unity: Add ImageCache.cs singleton for card art Sprite caching (#50)

unity-client/Assets/Scripts/Services/ImageCache.cs

@@ -0,0 +1,55 @@
+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+using UnityEngine.Networking;
+
+namespace CommanderAILab.Services
+{
+    /// <summary>
+    /// Singleton texture cache for card images.
+    /// Downloads and caches Scryfall card art as Sprite objects.
+    /// </summary>
+    public class ImageCache : MonoBehaviour
+    {
+        public static ImageCache Instance { get; private set; }
+
+        private readonly Dictionary<string, Sprite> cache = new();
+        [SerializeField] private int maxCacheSize = 500;
+
+        private void Awake()
+        {
+            if (Instance != null && Instance != this) { Destroy(gameObject); return; }
+            Instance = this;
+            DontDestroyOnLoad(gameObject);
+        }
+
+        public void GetSprite(string url, System.Action<Sprite> callback)
+        {
+            if (string.IsNullOrEmpty(url)) { callback?.Invoke(null); return; }
+            if (cache.TryGetValue(url, out var cached)) { callback?.Invoke(cached); return; }
+            StartCoroutine(DownloadImage(url, callback));
+        }
+
+        private IEnumerator DownloadImage(string url, System.Action<Sprite> callback)
+        {
+            using var request = UnityWebRequestTexture.GetTexture(url);
+            yield return request.SendWebRequest();
+
+            if (request.result != UnityWebRequest.Result.Success)
+            {
+                Debug.LogWarning($"[ImageCache] Failed to load {url}: {request.error}");
+                callback?.Invoke(null);
+                yield break;
+            }
+
+            var tex = DownloadHandlerTexture.GetContent(request);
+            var sprite = Sprite.Create(tex, new Rect(0, 0, tex.width, tex.height), Vector2.one * 0.5f);
+
+            if (cache.Count >= maxCacheSize) cache.Clear();
+            cache[url] = sprite;
+            callback?.Invoke(sprite);
+        }
+
+        public void ClearCache() => cache.Clear();
+    }
+}