Publishing reliability

Reliability is a trust property, not just an engineering one. Signal is built so a flaky network or a dying process can't turn into a double-post or a silently dropped post.

An atomic claim guarantees only one scheduler tick can publish a given item. Transient errors retry with backoff. If a publish is interrupted after the claim, the item is surfaced as a stale claim for deliberate, manual recovery instead of being blindly re-sent.