internal/chain holds typed Build, Rollout and Assignment and a Backend with no delete or purge, in memory and on the bus. The builds and rollouts buckets are asserted with history 64, no age and values up to just under a mebibyte; the assignments bucket gets the same, keeping its marker age because a stream cannot be made to disallow message ages. Migration 0104 makes the one-row chain_moved switch. The controller keeps no consolidated schema, so 0104 is the whole schema change. Nothing reads the buckets yet.