Introduce a service-owned download worker that offloads the full
download-and-finalize pipeline to DownloadService on Android, keeping
downloads alive independently of the Flutter UI process.
Key changes:
- Extract SAF download logic from MainActivity into SafDownloadHandler
- Add NativeDownloadFinalizer for Kotlin-side decryption, format
conversion, metadata embedding, ReplayGain, post-processing, and
history persistence
- Extend DownloadService with native queue management (start, pause,
resume, cancel) using coroutine-based worker with AtomicFile snapshots
- Add Dart-side orchestration: snapshot polling, run-id correlation,
adoption on app restart, and fallback to Dart queue
- Forward embedReplayGain, tidalHighFormat, and postProcessingEnabled
through Go backend DownloadRequest struct
- Add nativeDownloadWorkerEnabled setting with UI toggle
- Make DownloadQueueLookup collections unmodifiable
- Add 16 Go supplement test files covering extension runtime, providers,
health checks, lyrics, metadata, HTTP utils, library scan, and more
- Add Dart model/utils test suite (test/models_and_utils_test.dart)
- Update settings.g.dart with deduplicateDownloads serialization
