Changes touching this path

• the relay store goes behind a RelayStorage seam, the SigV4 driver moves to its own crate, and an object-store relay is a second deployable the CLI never ships (#38) 330df915 · dbf3dbe6…
• 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…diff

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