Add GitHub Actions CI: auto-build for Windows + macOS

- Windows: VST3 + Standalone (x64, MSVC 2022)
- macOS: VST3 + AU + Standalone (Universal Binary: arm64 + x86_64)
- Auto-release: push a tag (e.g. v1.2) to create GitHub Release with all artifacts
- Added AU format to CMakeLists.txt (macOS Audio Unit)
- Builds on every push to main and on PRs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -13,7 +13,7 @@ juce_add_plugin(InstaDrums
     NEEDS_MIDI_OUTPUT TRUE
     PLUGIN_MANUFACTURER_CODE Inst
     PLUGIN_CODE Idrm
-    FORMATS VST3 Standalone
+    FORMATS VST3 AU Standalone
     PRODUCT_NAME "InstaDrums"
     COPY_PLUGIN_AFTER_BUILD FALSE
 )