fuzz: model chanmon mempool mining

Route chanmon broadcasts through an explicit harness mempool so relay,
mining, wallet updates, and chain delivery share one path. This lets
splice, anchor, and claim transactions enter the mempool before mining.

On restart, sync loaded monitors and managers from their own persisted
best blocks so raw monitors catch up without rewinding ChannelManager
state. Cap modeled mining before unresolved HTLC timeout deadlines
and use the LDK anti-reorg depth for setup confirmations.

Author: joostjager