test: add JUnit 5 harness

pom.xml

@@ -22,6 +22,21 @@
     </repository>
   </repositories>
 
+  <dependencyManagement>
+    <dependencies>
+      <!-- The BOM keeps the jupiter engine and the platform launcher on matching
+           versions. Left to transitive resolution they drift apart, and a mismatched
+           launcher fails to discover tests rather than failing the build. -->
+      <dependency>
+        <groupId>org.junit</groupId>
+        <artifactId>junit-bom</artifactId>
+        <version>5.11.3</version>
+        <type>pom</type>
+        <scope>import</scope>
+      </dependency>
+    </dependencies>
+  </dependencyManagement>
+
   <dependencies>
     <dependency>
       <groupId>io.papermc.paper</groupId>
@@ -29,6 +44,12 @@
       <version>26.2.build.92-stable</version>
       <scope>provided</scope>
     </dependency>
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter</artifactId>
+      <!-- Version comes from junit-bom above. -->
+      <scope>test</scope>
+    </dependency>
   </dependencies>
 
   <build>
@@ -39,5 +60,15 @@
         <filtering>true</filtering>
       </resource>
     </resources>
+    <plugins>
+      <!-- Pinned because the version Maven bundles by default decides whether JUnit 5
+           tests are discovered at all. An older default would skip the whole suite and
+           still report BUILD SUCCESS, which is the one failure this project can't see. -->
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-surefire-plugin</artifactId>
+        <version>3.5.4</version>
+      </plugin>
+    </plugins>
   </build>
 </project>