Add focused Maven build CI

.github/workflows/maven.yml

@@ -0,0 +1,40 @@
+name: Maven build
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+permissions:
+  contents: read
+
+concurrency:
+  group: maven-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    runs-on: ubuntu-24.04
+    timeout-minutes: 20
+    steps:
+      - name: Check out sources
+        uses: actions/checkout@11d5960a326750d5838078e36cf38b85af677262 # v4
+      - name: Require Maven project
+        run: |
+          if [ ! -f pom.xml ]; then
+            echo '::error::pom.xml is missing. Merge the Maven project from PR #4 before enabling this build.'
+            exit 1
+          fi
+      - name: Set up Java
+        uses: actions/setup-java@cf277c60eb25467037889841efdb72551f06f6c3 # v4
+        with:
+          distribution: temurin
+          java-version: '17'
+          cache: maven
+      - name: Set up Maven
+        uses: stCarolas/setup-maven@d6af6abeda15e98926a57b5aa970a96bb37f97d1 # v5
+        with:
+          maven-version: '3.9.11'
+      - name: Compile and verify
+        run: mvn --batch-mode --no-transfer-progress clean verify