feat: add cross-platform CI for Linux, macOS, and Windows

- Split build.yml into lint, test (matrix: 3 OS), build-android, and
  build-desktop (matrix: 3 OS with native packaging) jobs
- Tests now run on ubuntu, macos, and windows to validate cross-platform
  compatibility
- Desktop distributions (DEB, DMG, MSI) built on their native OS and
  uploaded as artifacts
- Split create-release.yml into create-release, deploy-android, and
  deploy-desktop jobs so desktop distributions are included in releases
- Android APK/AAB builds and Quartz publishing remain on Linux

https://claude.ai/code/session_016dxFUErY4P6WRir4xxhCEr

.github/workflows/build.yml

@@ -1,4 +1,4 @@
-name: Test/Build Android
+name: Test/Build
 
 on:
   pull_request:
@@ -7,10 +7,9 @@ on:
     branches: [main]
 
 jobs:
-  build:
+  lint:
     runs-on: ubuntu-latest
-    timeout-minutes: 30
-
+    timeout-minutes: 15
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
@@ -24,20 +23,84 @@ jobs:
       - name: Cache gradle
         uses: actions/cache@v4
         with:
-          path: ~/.gradle/caches
-          key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*') }}
+          path: |
+            ~/.gradle/caches
+            ~/.gradle/wrapper
+          key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*', '**/gradle-wrapper.properties') }}
           restore-keys: |
             ${{ runner.os }}-gradle-
 
       - name: Linter (gradle)
         run: ./gradlew spotlessCheck
 
+  test:
+    needs: lint
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 30
+    defaults:
+      run:
+        shell: bash
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up JDK 21
+        uses: actions/setup-java@v4
+        with:
+          distribution: 'zulu'
+          java-version: 21
+
+      - name: Cache gradle
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.gradle/caches
+            ~/.gradle/wrapper
+          key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*', '**/gradle-wrapper.properties') }}
+          restore-keys: |
+            ${{ runner.os }}-gradle-
+
       - name: Test (gradle)
         run: ./gradlew test --no-daemon
 
       - name: Android Test Report
         uses: asadmansr/android-test-report-action@v1.2.0
-        if: ${{ always() }} # IMPORTANT: run Android Test Report regardless
+        if: ${{ always() && matrix.os == 'ubuntu-latest' }}
+
+      - name: Upload Test Results
+        uses: actions/upload-artifact@v4
+        if: ${{ always() && matrix.os == 'ubuntu-latest' }}
+        with:
+          name: Test Reports
+          path: amethyst/build/reports
+
+  build-android:
+    needs: test
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up JDK 21
+        uses: actions/setup-java@v4
+        with:
+          distribution: 'zulu'
+          java-version: 21
+
+      - name: Cache gradle
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.gradle/caches
+            ~/.gradle/wrapper
+          key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*', '**/gradle-wrapper.properties') }}
+          restore-keys: |
+            ${{ runner.os }}-gradle-
 
       - name: Build APK (gradle)
         run: ./gradlew assembleDebug
@@ -75,9 +138,54 @@ jobs:
           name: Compose Reports
           path: amethyst/build/compose_compiler
 
-      - name: Upload Test Results
+  build-desktop:
+    needs: test
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: ubuntu-latest
+            task: packageDeb
+            artifact-name: Desktop Linux DEB
+            artifact-path: desktopApp/build/compose/binaries/main/deb/*.deb
+          - os: macos-latest
+            task: packageDmg
+            artifact-name: Desktop macOS DMG
+            artifact-path: desktopApp/build/compose/binaries/main/dmg/*.dmg
+          - os: windows-latest
+            task: packageMsi
+            artifact-name: Desktop Windows MSI
+            artifact-path: desktopApp/build/compose/binaries/main/msi/*.msi
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 30
+    defaults:
+      run:
+        shell: bash
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up JDK 21
+        uses: actions/setup-java@v4
+        with:
+          distribution: 'zulu'
+          java-version: 21
+
+      - name: Cache gradle
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.gradle/caches
+            ~/.gradle/wrapper
+          key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*', '**/gradle-wrapper.properties') }}
+          restore-keys: |
+            ${{ runner.os }}-gradle-
+
+      - name: Build Desktop Distribution
+        run: ./gradlew :desktopApp:${{ matrix.task }}
+
+      - name: Upload Desktop Distribution
         uses: actions/upload-artifact@v4
         with:
-          name: Test Reports
-          path: amethyst/build/reports
-
+          name: ${{ matrix.artifact-name }}
+          path: ${{ matrix.artifact-path }}