feat: a machine answers for the past it wrote before it had a key - #103
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A history written before this machine had a key was folded into the chain either way, so what it signs now covers those bytes. What it did not have was anything answering for those segments on their own: a reader starting anywhere but the beginning had nothing to check them against, and the machine's own past was the one stretch of its history that stood on the say-so of a later signature. On the first write after the key exists, every closed segment of its own that nothing answers for gets a signature of its own, over the chain that segment really closes. The pass costs one reading of the history, once, because the round after it finds nothing left to sign. The place it belongs was already there: the loop that folds the unsigned past into the chain is holding exactly the tip each of those signatures has to attest. Only where nothing answers for it yet. A signature already in place attests a history, and writing another over it would bless bytes that history never covered — so one that stands is left exactly as it stands, whether it answers or not.
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A history written before this machine had a key was folded into the chain either way, so what it signs now covers those bytes — but nothing answered for those segments on their own, and a reader starting anywhere but the beginning had nothing to check them against. On the first write after the key exists, every closed segment of its own that nothing answers for gets a signature over the chain that segment really closes; the pass costs one reading of the history, once. Only where nothing answers for it yet: a signature already in place attests a history, and writing another over it would bless bytes that history never covered, so one that stands is left as it stands.