Gate r3 finding 2c: when _summary_placement merged the handoff into
tail[0] and that row is an assistant(tool_calls), the dedup's
move-carrier-to-window-start step popped it away from its tool results
(broken pairing) and still left an assistant row first.
Snapshot the window before the dedup filter; if the filter removed a row
and the carrier is a tool-call row (_template_visible_role is None),
restore the snapshot and skip the dedup this cycle. The output then
matches the pre-stack layout for that shape; the standalone-carrier
move-and-flip path is unchanged.
2667c960d5