Changes touching this path

• the relay contract is asserted once and both backends run it, the mailbox choreography moves behind a blanket impl a backend cannot override, and the fs and bucket acks stop disagreeing about which of them was right (#1164) eb659a91 · dbf3dbe6…
• a bucket-backed relay reaps, because the anchor is when the bytes last landed in the store and the store is what answers it, and the reap itself moves behind the seam so the standing exemption is one rule both backends run (#1166) 39e1e1eb · dbf3dbe6…diff

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