A decoded, committed transaction delivered to a Sink.

changes is typed Enumerable.t(), not a list. C1 delivers a list, but the type is the day-one contract: C3's spill/batch work must be able to switch a transaction's changes to a lazy, single-pass, disk-backed enumerable — as replicant does — without silently breaking a sink. A sink MUST NOT call length/1 (or anything else requiring more than one pass) on changes; a single-pass enumerable would raise or return a wrong count on a second pass, and a length/1 habit that happens to pass every capstan test today would silently consume a single-pass reader later. Enumerate once, streaming (Enum.reduce/3, Enum.each/2, for, or Stream pipelines that consume the enumerable exactly once).

position is the processed GTID set INCLUDING this transaction — see Capstan.Position. gtid is this transaction's own identity, "uuid:gno".