Function calculate_td_by_number 

pub fn calculate_td_by_number<P>(provider: &P, num: BlockNumber) -> Result<U256>
where
    P: BlockReader,

Imports history from ERA files. Calculates the total difficulty for a given block number by summing the difficulty of all blocks from genesis to the given block.

Very expensive - iterates through all blocks in batches of 1000.

Returns an error if any block is missing.