Add CI workflow for Gradle and Java 21

.github/workflows/main.yml

@@ -0,0 +1,62 @@
+name: CI (Gradle • Java 21)
+
+on:
+  push:
+    branches: [ "**" ]
+  pull_request:
+    branches: [ "**" ]
+
+jobs:
+  build-test:
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up JDK 21
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: '21'
+
+      - name: Setup Gradle
+        uses: gradle/actions/setup-gradle@v3
+
+      # Sometimes repo line endings can drop the exec bit; this makes gradlew runnable.
+      - name: Ensure gradlew is executable
+        run: chmod +x ./gradlew
+
+      - name: Build & test
+        run: ./gradlew clean test build --no-daemon
+
+      - name: Generate coverage (JaCoCo)
+        run: ./gradlew jacocoTestReport --no-daemon
+
+      - name: Upload test results
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: test-results
+          path: |
+            **/build/test-results/**
+            **/build/reports/tests/**
+
+      - name: Upload coverage report
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: coverage
+          path: |
+            **/build/reports/jacoco/**
+            **/build/jacoco/**
+
+      - name: Upload app jar
+        if: success()
+        uses: actions/upload-artifact@v4
+        with:
+          name: app
+          path: |
+            **/build/libs/*.jar
+            !**/*-plain.jar