SIGROK-derived PCAPs have relative timestamps (0, 1.37, 15.02) rather
than epoch time. The _REAL_TS_MIN check failed, so close_session() set
ended to wall-clock (same as started → duration 0s). Now close_session
accepts an optional duration_secs param; the import handler passes
last_ts - first_ts so the session gets the correct capture duration.