When a message directly wakes its recipient, the broker emits `sent`
then `delivered` for the same row id ~instantly, rendering two
near-identical lines on the FL0W timeline. Track each recent `sent`
row and, when its `delivered` lands within COLLAPSE_SECS (3s), upgrade
that row in place (arrow → green ✓, title "sent + delivered") instead
of adding a second line. A delivery that arrives later (recipient was
busy) still renders as its own row, so genuine delivery latency stays
visible.
`Sent` and `Delivered` share the same broker row id
(dashboard_events.rs), so matching is exact. The recentSent map is
bounded to 256 entries. Reply threading is unaffected (msgRowMap still
resolves the surviving row).
Closes#1472.
a2fe3f2e01