Function write_loop 

pub async fn write_loop(
    storage: Arc<dyn StoragePlugin>,
    registry: Arc<PvRegistry>,
    rx: Receiver<PvSample>,
    shutdown: Receiver<bool>,
    flush_period: Duration,
)

Background writer task — drains samples and writes to storage.

Thin wrapper that fills WriteLoopConfig from Default and the caller-supplied flush_period. Production callers use this form; the test suite uses write_loop_with_config directly to dial timeouts down.