fix(mods): 加载器 mod 仅在正版验证的独立服务器上签发绑定码，README 标明只用 limbo token 及其风险

plugins/shared/src/main/java/best/lolicon/felis/link/ModLinkPolicy.java

@@ -0,0 +1,24 @@
+package best.lolicon.felis.link;
+
+/**
+ * ModLinkPolicy is what the Fabric, Forge and NeoForge mods say when they refuse
+ * {@code /link}. A mod mints a link code for the UUID its server reports, and only an
+ * online-mode server has checked that UUID with Mojang. Behind a proxy the backend runs
+ * offline-mode and the proxy's own {@code /link} already serves every backend; on a
+ * cracked server the UUID is whatever the client claims, so a code minted there would
+ * let anyone link someone else's Minecraft account. The mods therefore link only on a
+ * standalone online-mode server.
+ */
+public final class ModLinkPolicy {
+    /** OFFLINE_REPLY is the chat line a player gets on an offline-mode server. */
+    public static final String OFFLINE_REPLY =
+            "此服务器未开启正版验证，不能在这里绑定 / This server runs with online-mode off, so /link is unavailable here.";
+
+    /** OFFLINE_LOG is the server-log line for the same refusal. */
+    public static final String OFFLINE_LOG =
+            "Felis link: refused /link because online-mode is off. The mod links only on a standalone "
+                    + "online-mode server; behind the Felis proxy, the proxy serves /link.";
+
+    private ModLinkPolicy() {
+    }
+}