A deterministic fallback summary replaced the older handoff in the transcript but never updated _previous_summary. The next compaction kept the stale in-memory summary and dropped the fallback row from its window, so the fallback's user asks, files and last dropped turns never reached the summarizer. Store the fallback body in _previous_summary the same way a normal summary is stored.
(cherry picked from commit 35417d2e1ffbb775c3eaff17b26623896afa56c1)
e46d4c0ade