Run checkstyle as separate CI job on Linux with latest Java

Move checkstyle out of the test phase and into a dedicated CI job.
This allows dependabot to update the checkstyle version and speeds
up test runs on Windows/macOS by not running checkstyle there.

Checkstyle 13+ requires Java 21+, so we run it on Java 24 on Linux.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: horgh

.github/workflows/checkstyle.yml

@@ -0,0 +1,17 @@
+name: Run checkstyle
+# Checkstyle 13+ requires Java 21+.
+on: [push, pull_request]
+permissions: {}
+jobs:
+  checkstyle:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+        with:
+          submodules: true
+          persist-credentials: false
+      - uses: actions/setup-java@f2beeb24e141e01a676f977032f5a29d81c9e27e # v5.1.0
+        with:
+          distribution: zulu
+          java-version: 24
+      - run: mvn checkstyle:check -B

pom.xml

@@ -127,6 +127,25 @@
                   </execution>
                 </executions>
             </plugin>
+            <!-- Checkstyle 13+ requires Java 21+. Run manually or via CI. -->
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-checkstyle-plugin</artifactId>
+                <version>3.6.0</version>
+                <configuration>
+                    <consoleOutput>true</consoleOutput>
+                    <configLocation>checkstyle.xml</configLocation>
+                    <suppressionsLocation>checkstyle-suppressions.xml</suppressionsLocation>
+                    <violationSeverity>warning</violationSeverity>
+                </configuration>
+                <dependencies>
+                    <dependency>
+                        <groupId>com.puppycrawl.tools</groupId>
+                        <artifactId>checkstyle</artifactId>
+                        <version>13.0.0</version>
+                    </dependency>
+                </dependencies>
+            </plugin>
             <plugin>
                 <artifactId>maven-javadoc-plugin</artifactId>
                 <version>3.12.0</version>
@@ -340,43 +359,5 @@
                 </plugins>
             </build>
         </profile>
-        <!-- Checkstyle 13+ requires Java 21+. -->
-        <profile>
-            <id>checkstyle-jdk21</id>
-            <activation>
-                <jdk>[21,)</jdk>
-            </activation>
-            <build>
-                <plugins>
-                    <plugin>
-                        <groupId>org.apache.maven.plugins</groupId>
-                        <artifactId>maven-checkstyle-plugin</artifactId>
-                        <version>3.6.0</version>
-                        <configuration>
-                            <consoleOutput>true</consoleOutput>
-                            <configLocation>checkstyle.xml</configLocation>
-                            <suppressionsLocation>checkstyle-suppressions.xml</suppressionsLocation>
-                            <violationSeverity>warning</violationSeverity>
-                        </configuration>
-                        <dependencies>
-                            <dependency>
-                                <groupId>com.puppycrawl.tools</groupId>
-                                <artifactId>checkstyle</artifactId>
-                                <version>13.0.0</version>
-                            </dependency>
-                        </dependencies>
-                        <executions>
-                            <execution>
-                                <id>checkstyle</id>
-                                <phase>test</phase>
-                                <goals>
-                                    <goal>check</goal>
-                                </goals>
-                            </execution>
-                        </executions>
-                    </plugin>
-                </plugins>
-            </build>
-        </profile>
     </profiles>
 </project>