Compaction swapped a session's in-memory history for a summary but left the full
transcript in storage, so resume() reloaded all of it -- on a long session, or
one switched to a smaller-context model, the rehydrated context overflowed the
model window and deadlocked the first post-resume send.
Persist a `_source="compaction"` marker (summary + watermark) on compaction;
resume rehydrates [summary] + [rows after the watermark] instead of the full
transcript. Full history stays in storage for /history, export, and audit;
markers are filtered from display, search, and export, and rewind/retry
truncation is floored at the marker so the summary's backing is never deleted.
The watermark and search filters count real transcript rows only. No migration.
Patrick Buckley