machineMemo property
FR-28 — opaque machine-memo bytes plus the purpose the user is shown.
Mutually exclusive with memo: one payment, one memo. Unlike the text memo — which only SEEDS an editable field — these bytes are not editable and not re-derivable from the form, so the screen carries them through every re-compose (a user editing the amount or the recipient does not silently drop the host's reference; that silent drop is the defect this FR exists to fix).
A memo needs a shielded recipient. If the user edits the recipient to a transparent address the send is refused typed, exactly as a text memo is — the bytes are never silently dropped to let the payment through.
Implementation
final WalletMachineMemo? machineMemo;