Export bookings whose cargo goes straight from customer truck to wagon
never get a warehouse GRN, so the load gate (assertExportReceivedWithGrn)
rejects them forever unless staff first flip exportHandoverMode on the
booking detail page, then come back and retry Load.
Add a Truck to Train button next to Load in the schedule workspace's
on-train list, shown for EXPORT bookings at the boarding yard. One click
sets exportHandoverMode=DIRECT_TO_TRAIN then loads the booking.
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