The reopen backfill guard `child.started_at >= parent.ended_at` compared against
the parent's CURRENT end boundary. A genuine markerless legacy reset child whose
parent was later reopened and re-ended has started_at earlier than that second
boundary, so it was no longer frozen with `_reset_from`; once end_reason cleared
it dropped out of /sessions as ephemeral — the multi-cycle gateway-peer shape the
issue describes. Comparing against the parent's started_at still rejects
children that predate the parent while keeping every earlier-boundary reset
child; the marker and source exclusions are unchanged.
Review finding: cycled parent's earlier reset child lost its `_reset_from` stamp and vanished from the session list on reopen.
teknium1
5ffdf823ae