Function spawn_receipt_writer 

pub fn spawn_receipt_writer(
    log: Arc<dyn ReceiptLog>,
    metrics: Arc<Metrics>,
) -> (Arc<dyn ReceiptSink>, StopHandle)

Spawn the single background task that owns the on-disk ReceiptLog and drains the receipt queue, returning the ReceiptSink the delivery path enqueues through plus the StopHandle that stops the task at graceful shutdown.

Decouples the audit write from the paid-delivery hot path (#803): the voucher-accept path does only a non-blocking ReceiptSink::record (a bounded try_send) as each voucher is accepted, while this task performs the actual append on the blocking pool. A slow or full disk can only fill the queue (and drop audit records, counted) — it can never delay delivery or serialize delivery on the receipt-log mutex. The writer is the only caller of ReceiptLog::append, so the log’s internal mutex sees no cross-stream contention.

On its stop signal (StopHandle::cancel, fired after the router has drained, so no further receipts are produced) the task flushes whatever is already enqueued and exits; complete StopHandle::shutdown within the shutdown deadline to preserve the audit tail. The StopHandle owns the stop token beside the task handle, so the join cannot hang on sinks that outlive shutdown — crate::handlers::client::ClientHandler holds one for the whole runtime.