Completed exchanges keep the shared wallet projection pinned to the old
module 3 result and the cancel guard refuses to roll the coins back, so a
retrained card could never reach modules 4 and 5. The new endpoint wipes
that user's exchange and payment history, returns the coins to the
commercial bank vault, and re-projects from the latest module 3 result in
the same transaction so the read-only context endpoints stay usable.
chenyuan