fuzz: model chanmon mempool mining

Add a modeled mempool and wallet-backed transaction relay to the
chanmon consistency target. Broadcast transactions are admitted through
explicit relay steps, mined by block commands, and then replayed to the
monitor and manager through the harness chain state.

Track confirmed UTXOs so mempool admission can reject duplicate or
unknown inputs, model direct RBF conflicts, and keep wallet UTXOs in
sync with confirmed change outputs.

Author: joostjager