Litestream Writable VFS

Litestream VFS now supports write operations and background hydration for faster cold starts. The writable VFS mode buffers writes locally and syncs to object storage periodically, enabling single-writer scenarios with eventual durability. Background hydration pulls the full database from S3 while serving reads remotely, then switches to the local copy when ready. These features power Fly.io's Sprites storage stack, where every Sprite boots with 100GB of durable storage in under a second by using Litestream SQLite with S3-backed block maps.