Add GitHub Actions CI: build Rust plugin + Compose UI

- Matrix build across Linux, macOS, Windows
- Rust: builds VST3 plugin lib + standalone binary
- Kotlin: builds Compose Desktop UI + attempts packaging
- Caches cargo registry and Gradle for faster runs
- Uploads build artifacts per platform

Author: VeldtJumper

.github/workflows/build.yml

@@ -0,0 +1,92 @@
+name: Build
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  rust-plugin:
+    name: Rust Plugin
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
+    defaults:
+      run:
+        working-directory: plugin
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Rust
+        uses: dtolnay/rust-toolchain@stable
+
+      - name: Cache cargo
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            plugin/target
+          key: ${{ runner.os }}-cargo-${{ hashFiles('plugin/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-cargo-
+
+      - name: Install ALSA dev (Linux)
+        if: runner.os == 'Linux'
+        run: sudo apt-get update && sudo apt-get install -y libasound2-dev
+
+      - name: Build VST3 plugin
+        run: cargo build --release
+
+      - name: Build standalone
+        run: cargo build --release --features standalone --bin compose-vst-standalone
+
+      - name: Upload plugin artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: plugin-${{ matrix.os }}
+          path: |
+            plugin/target/release/libcompose_vst_plugin.so
+            plugin/target/release/libcompose_vst_plugin.dylib
+            plugin/target/release/compose_vst_plugin.dll
+            plugin/target/release/compose-vst-standalone*
+          if-no-files-found: ignore
+
+  compose-ui:
+    name: Compose UI
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
+    defaults:
+      run:
+        working-directory: ui
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up JDK 21
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: 21
+
+      - name: Setup Gradle
+        uses: gradle/actions/setup-gradle@v4
+
+      - name: Generate wrapper (if missing)
+        run: gradle wrapper
+
+      - name: Build
+        run: ./gradlew build
+
+      - name: Package
+        run: ./gradlew packageReleaseDistributionForCurrentOS
+        continue-on-error: true
+
+      - name: Upload UI artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: ui-${{ matrix.os }}
+          path: ui/build/compose/binaries/main-release/
+          if-no-files-found: ignore