feat: Add At-Least-Once Fan-Out With Redelivery On Nack - #6
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Each subscriber gets a private inbox. ack settles that copy. nack redelivers it to the same subscriber only. Bounded by maxDeliveryCount. Restore DeadLetterQueue exports dropped from the package index.
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Topic fan-out was still fire-and-forget. If a handler failed, that subscriber never saw the message again. The work queue already retries a single competing consumer. This adds the same guarantee to pub/sub.
ReliableBroker gives each subscriber a private inbox. Publish still copies to everyone, but ack and nack are per copy. A nack (or a thrown handler) comes back to the same subscriber with deliveryCount bumped and redelivered set. Peers who already acked are left alone. Retries stop at maxDeliveryCount so a poison payload cannot spin inside publish. Optional prefetch isolates a slow consumer without stalling the others.
Also puts the DeadLetterQueue exports back on the package index. The work-queue merge had dropped them and the existing DLQ tests could not typecheck.
Closes #5.