Light client forkfulness

[adlerjohn]

An optimistic rollup on top of Celestia needs to be able to handle forks in the rollup. The reason for this is that multiple rollup blocks at the same height can be posted, and only after the fraud proof time window has passed can a single fork be selected by the fork choice rule.

The intuition here is that a "bridge-finalized" boundary progresses, and selects the first rollup block posted at a particular height that hasn't been proven invalid.

Refs:

• https://medium.com/offchainlabs/how-arbitrum-rollup-works-39788e1ed73f