feat: implement LicenseGate for feature checking

- Thread-safe AtomicReference-based license holder
- Defaults to open mode (all features enabled) when no license loaded
- Runtime license loading with feature/limit queries
- Unit tests for open mode and licensed mode

cameleer3-server-core/src/main/java/com/cameleer3/server/core/license/LicenseGate.java

@@ -0,0 +1,35 @@
+package com.cameleer3.server.core.license;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+public class LicenseGate {
+
+    private static final Logger log = LoggerFactory.getLogger(LicenseGate.class);
+
+    private final AtomicReference<LicenseInfo> current = new AtomicReference<>(LicenseInfo.open());
+
+    public void load(LicenseInfo license) {
+        current.set(license);
+        log.info("License loaded: tier={}, features={}, expires={}",
+                license.tier(), license.features(), license.expiresAt());
+    }
+
+    public boolean isEnabled(Feature feature) {
+        return current.get().hasFeature(feature);
+    }
+
+    public String getTier() {
+        return current.get().tier();
+    }
+
+    public int getLimit(String key, int defaultValue) {
+        return current.get().getLimit(key, defaultValue);
+    }
+
+    public LicenseInfo getCurrent() {
+        return current.get();
+    }
+}