signProvenOffer
Sign the fallible unshielded offer on a PROVEN contract-call transaction.
The offer that funds a receiveUnshielded is attached unsigned at assembly and signed here, after proving — proving rewrites the call's erased serialization, so a signature made earlier would fail the node's verification (error 175).
Return
the signed transaction hex, or a {"error":...} payload / null on failure.
Parameters
provenTxHex
the proven transaction hex (from the proof provider).
nightPrivateKeyHex
the wallet's NIGHT signing key hex (32 bytes).