This seam sits between closing out one task and closing out the tranche that contained it. It exists because a tranche's history is only honest if each task's history was completed first.
What crosses — the set of merged pull requests, each carrying its own provenance record. That record is what proves a task was genuinely closed out rather than merely merged. Alongside it: a note on the standing lessons thread for any serious finding that was raised in review and merged anyway, so a consciously accepted deviation is available as a pattern later; and a follow-up issue for every finding that the product's specification has gone stale, so the gap outlives the task that found it.
The hand-off is malformed when — a merged pull request carries no provenance record. That means the task's close-out never ran, and the tranche's retrospective would then be assembled from a gap rather than a fact. It is malformed in the same way when an accepted deviation left no note, or when a stale-specification finding left no follow-up: in each case something real happened and nothing durable records it.
What it does not carry — permission to proceed on partial evidence. A partial close-out is worse than none, because it produces a record that looks complete and is not. It also carries no new judgement: nothing here re-opens, re-reviews or re-decides a merged task; both roles assemble from facts, and neither authors one.
How it physically runs — the carrier is the merged pull requests themselves, each with its provenance comment, plus the standing lessons thread and any follow-up issues. Tranche close-out reads them and stops if one is missing, flagging exactly which task's record is incomplete rather than working around it. The retrospective it then writes is a structured projection of those facts — assembled from merged work, recorded lessons and the decisions of the period, never generated from memory.