Steam 游戏封面三级降级库：内存缓存（跨视图共享）→ 持久化缓存（跨会话，GM_setValue，LRU 500）→ CDN fallback 链（Akamai 优先 → Cloudflare 兜底）→ Steam appdetails API 兜底 → SVG placeholder。供 SGLV 系列脚本（game-library-viewer / wishlist / friend / badges ...）@require 引用，复用降级逻辑。不可单独使用。
This script should not be not be installed directly. It is a library for other scripts to include with the meta directive // @require https://update.greasyfork.org/scripts/589926/1909126/SGLV%20%E5%B0%81%E9%9D%A2%E4%B8%89%E7%BA%A7%E9%99%8D%E7%BA%A7%20%28Library%29.js
