ci: build and test the main ports on GitHub Actions

Build every push and pull request for 26.2, 1.21.11 and 1.20.1, each with
the JDK it needs, and run the JUnit suites of the shared data layer, so a
broken port or a regression in the common code is caught before release.

Author: marcsanz-dev

.github/workflows/build.yml

@@ -0,0 +1,38 @@
+name: Build
+
+on:
+  push:
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  build:
+    name: ${{ matrix.module }} (JDK ${{ matrix.java }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - module: multiloader-26.2   # Minecraft 26.2 — Fabric + NeoForge
+            java: '25'
+          - module: multiloader        # Minecraft 1.21.11 — Fabric + NeoForge
+            java: '21'
+          - module: multiloader-1.20.1 # Minecraft 1.20.1 — Fabric + Forge
+            java: '21'
+    defaults:
+      run:
+        working-directory: ${{ matrix.module }}
+    steps:
+      - uses: actions/checkout@v5
+
+      - uses: actions/setup-java@v5
+        with:
+          distribution: temurin
+          java-version: ${{ matrix.java }}
+          cache: gradle
+          cache-dependency-path: |
+            ${{ matrix.module }}/**/*.gradle*
+            ${{ matrix.module }}/**/gradle-wrapper.properties
+
+      - name: Build and run tests
+        run: bash ./gradlew build --no-daemon --stacktrace