Function mine_block 

pub fn mine_block(
    block: Block,
    max_attempts: u64,
) -> Result<(Block, MiningResult), ConsensusError>

MineBlock: ℬ × ℕ → ℬ × {success, failure}

Attempt to mine a block by finding a valid nonce:

1. Try different nonce values
2. Check if resulting hash meets difficulty target
3. Return mined block or failure