Return/change from overpaid fees. Empty when the mint defers change (async/onchain melts).
NUT-08 outputs retained for deferred-change recovery (onchain, NUT-23 prefer_async). Empty
when change is populated — no recovery needed. Otherwise pair with the polled quote's
change via wallet.createMeltChangeProofs().
If false, the proofs have not been invalidated and the payment can be tried later again with the same proofs.
Response after melting proofs.