When creating a new spend, if coin outpoints are not provided,
then coins will be selected automatically.
This automatic selection is such that the transaction fee is
minimized, taking into account the cost of creating any
change output now and the cost of spending it in the future.
If change is added, it must reduce the transaction waste and
be above the dust threshold. This same policy is applied also
in the case of manual coin selection, replacing the previous
logic for determining the change amount. This ensures that
creating a spend with auto-selection and another with manual
selection using the same auto-selected coins will give the
same change amount.
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