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-app/src/test/java/com/cameleer3/server/core/license/LicenseGateTest.java

@@ -0,0 +1,41 @@
+package com.cameleer3.server.core.license;
+
+import org.junit.jupiter.api.Test;
+
+import java.time.Instant;
+import java.time.temporal.ChronoUnit;
+import java.util.Map;
+import java.util.Set;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class LicenseGateTest {
+
+    @Test
+    void noLicense_allFeaturesEnabled() {
+        LicenseGate gate = new LicenseGate();
+        // No license loaded -> open mode
+
+        assertThat(gate.isEnabled(Feature.debugger)).isTrue();
+        assertThat(gate.isEnabled(Feature.replay)).isTrue();
+        assertThat(gate.isEnabled(Feature.lineage)).isTrue();
+        assertThat(gate.getTier()).isEqualTo("open");
+    }
+
+    @Test
+    void withLicense_onlyLicensedFeaturesEnabled() {
+        LicenseGate gate = new LicenseGate();
+        LicenseInfo license = new LicenseInfo("MID",
+                Set.of(Feature.topology, Feature.lineage, Feature.correlation),
+                Map.of("max_agents", 10, "retention_days", 30),
+                Instant.now(), Instant.now().plus(365, ChronoUnit.DAYS));
+        gate.load(license);
+
+        assertThat(gate.isEnabled(Feature.topology)).isTrue();
+        assertThat(gate.isEnabled(Feature.lineage)).isTrue();
+        assertThat(gate.isEnabled(Feature.debugger)).isFalse();
+        assertThat(gate.isEnabled(Feature.replay)).isFalse();
+        assertThat(gate.getTier()).isEqualTo("MID");
+        assertThat(gate.getLimit("max_agents", 0)).isEqualTo(10);
+    }
+}