Introduce compute_shuffling_lookahead_start_slot helper

[jihoonsong]

shuffling_decision_slot is the next slot to dependent_slot, except for the first two epochs. For GENESIS_EPOCH and GENESIS_EPOCH + MIN_SEED_LOOKAHEAD, it's equal to dependent_slot, which is GENESIS_SLOT. I've found this useful for advancing dependent state just as necessary as to advance proposer lookahead.

As for the concern of advancing state, my interpretation of this chunk of code is to fetch and use cached proposer lookahead. That said, I'm open to a better expression than what we have.

I stole the name from is_previous_slot_payload_decision. Other names that I considered are:

• compute_shuffling_processed_slot
• compute_shuffling_known_slot
• compute_shuffling_available_slot
• compute_shuffling_determined_slot

IMO, known and available are ambiguous because it can be any slot in current and past epochs. It should mean the first slot of the previous epoch.

Another way is introducing compute_shuffling_lookahead_epoch and doing compute_start_slot_at_epoch(compute_shuffling_lookahead_epoch(epoch)). But I think adding a helper returning the start slot of lookahead epoch brings us slightly more value.

[jtraglia]

Naming things is tough. I'm thinking we should do:

• compute_lookahead_start_slot (preferred)
• compute_shuffling_lookahead_start_slot

I've looked through the usages below & I believe this to be fine. I do not believe it is necessary to mention "shuffling" here.

[jihoonsong]

I'd go with compute_shuffling_lookahead_start_slot. I believe it should mention shuffling; otherwise, it's ambiguous which lookahead is being referred to. For instance, the lookahead for proposer has changed post-Fulu. See #5374 for more context.