Function download_object 

pub async fn download_object<E, C>(
    client: Arc<Client>,
    bucket: BucketId,
    key: String,
    dest_path: PathBuf,
    request_id: String,
    channel: &E,
    registry: TransferRegistryHandle,
    lock_registry: Arc<LockRegistry>,
    cancel_rx: Receiver<()>,
    profile_id: ProfileId,
    lock_ttl_secs: u64,
    log: NotificationLogHandle,
    os_notifier: &OsNotifier<C>,
) -> Result<(), AppError>
where
    E: EventEmitter,
    C: OsNotifyChannel,

Stream-download key from bucket and write it atomically to dest_path.

Parameters

• client — authenticated S3 client for the profile’s region.
• bucket — target bucket name.
• key — S3 object key.
• dest_path — local destination path (final, not .partial).
• request_id — UUID v4 string from TransferRegistry::register.
• channel — Tauri AppHandle or MockChannel for event emission.
• registry — shared TransferRegistryHandle for state updates.
• lock_registry — resource lock registry; this function acquires and releases the lock.
• cancel_rx — oneshot receiver; resolved when the caller calls TransferRegistry::cancel.
• profile_id — used to build the LockScope.
• lock_ttl_secs — TTL for the acquired lock.
• log — in-app notification log for terminal-state notifications.
• os_notifier — OS notification bridge (settings-gated internally).