Join recordings of one flight that was stopped and restarted #237

Is your feature request related to a problem? Please describe.
The client said that if a recording is stopped during a flight and restarted, it is still one flight, and that parts should be joined by timestamp. Today pilot_id and flight_id come from the logger (#211) or from the file name (#228, P001_T3), so a stopped and restarted recording would be counted as two flights. Two flights from one trial would be weighted twice in a pilot's average and would each look like a short flight.

Describe the solution you'd like
A way to say that several recordings are one flight, applied before feature engineering.

• Define the rule: recordings by the same pilot whose timestamps are contiguous (gap under a threshold) belong to one flight. Or take an explicit mapping file if the gap alone is ambiguous.
• Keep one flight_id for the merged flight and report which files were merged.
• Do not interpolate across the gap. Flag it the same way Trim each flight to the airborne portion and segment it into legs #229 flags a sample gap.
• Make sure trimming and leg segmentation (Trim each flight to the airborne portion and segment it into legs #229) run on the merged flight, so a gap in the middle of a leg does not split it.

Additional context
Open question for the client: how long a pause counts as the same flight, and whether the pilot flew continuously during the pause or the simulator was paused. Notes in docs/client_meeting_notes.md.