universalAckFlow

The causal-stability watermark: the highest sequence number that every currently known peer has acknowledged. Advances monotonically — it never decreases.

A newly-joined peer receives a QuiltMessage.FullState that already reflects any compacted history, so it does not need to acknowledge old deltas before the watermark can advance. Consequently a late-joiner's absence from ackedThrough does not drag the watermark backward: the flow stays at its last value until the peer actually acks (or is evicted). Eviction of a lagging peer may legitimately raise the watermark.

Emits 0L until at least one delta has been universally acknowledged.