Returns to the original model (data-model §2.8: a closed deal produces a
Movement and, optionally, a Plantare): the given/received Movement plus,
when they came with it, the Sale (informational payment note) and the
Plantare (return promise), all sharing the counterparty. The Movement
carries the promise via the plantare_id column the schema had reserved.
'Gave all' (one tap, the natural path for shrubs/whole plants) moves the
lot's whole quantity, leaves an empty jar (0, same unit), clears
abundance, and returns the lot to private — withdrawing its market offer
on the next publish cycle, price cleared with it.
New enum HandoverDirection; tests in test/data/handover_test.dart.