Dispatches one delivered batch into the registered processors.

Every message is acked or retried exactly once:

• a body that is not a readable envelope → retry(), because a message the consumer cannot route is a configuration problem and acking would discard it permanently and silently;
• a name with no registered processor → retry(), same reason;
• attempts past the envelope's maxAttempts → ack() without running the processor, since the caller asked for the job to stop;
• a processor that throws → retry(), leaving the queue's own max_retries and dead-letter configuration to decide what happens next;
• a processor that resolves → ack().