First block of #65: the state machine, the platform download engine, a
single-song download from the song context menu, the downloaded indicator in
song lists, and local-first playback.
- `song_downloads`, keyed `(sourceId, songId)` with CASCADE on the source, and
the 16 -> 17 migration.
- `DownloadEngine` over `android.app.DownloadManager`, with a fake for tests.
Files land in app-specific external storage at
`.../Music/downloads/<sourceId>/<songId>`, plus a `.nomedia` so media scanners
on pre-Android-10 devices leave them alone.
- `DownloadRepository` enqueues, mirrors progress by polling, cancels, deletes
and reconciles on start and on source change: a queued row whose engine entry
is gone is started again, a completed row whose file is gone becomes failed
rather than being resurrected, and files with no row are swept.
- Playback prefers the downloaded file: the media item build takes the local URI
when the download is complete and leaves it unclipped.
- Deleting a download reloads the player window at its position, so the item
falls back to streaming instead of failing mid-read.
- Deleting a source cancels its downloads and removes their files.
Downloads are the system's own notifications and stay off metered networks for
now. Artwork, bulk downloads and the management screen are later blocks.
