aw-sync: rowid upgrade bootstrap does not recover backfills lost under the old timestamp cursor #793

Follow-up from #789 (review thread on the upgrade bootstrap).

Gap. On the first pull after upgrading to the rowid cursor (stored_rowid == 0 and the destination already has events), #789 seeds the cursor at the source's current max rowid and runs one last timestamp-cursor pass. A backfilled event (old timestamp, new rowid) that landed in the source before that first rowid pull sits below both boundaries: the max-rowid seed marks it consumed, and the timestamp pass starts after the destination's newest event. It is never copied.

Scope. This is not a regression. The same events were already lost under the old destination-as-marker cursor (#696); #789 stops new losses but does not recover old ones. My earlier description on the review thread ("only if the timestamp pass fails") was too narrow: the miss happens even when that pass succeeds.

Possible fix. A one-time, opt-in reconcile on the bootstrap pass (or an explicit aw-sync --reconcile flag): scan source history, then insert events missing from the destination, matched by (timestamp, duration, data). Heartbeat-merged events complicate the matching, so this needs a design decision before anyone implements it. It is not needed to merge #789.