Function select_claim_prefix 

pub fn select_claim_prefix(
    candidates: &[ClaimCandidate],
    boundary: QueuedWorkClaimBoundary,
    max_batches: usize,
) -> usize

Select the prefix of candidates that a single claim may take.

Returns the number of leading candidates to claim (0 means no claim):

• An QueuedWorkClaimBoundary::ActiveTurnCheckpoint boundary only admits work whose head batch is DeliveryPolicy::EarliestSafeBoundary.
• An SlotPolicy::Exclusive head claims exactly one batch.
• A SlotPolicy::Join head extends through immediately following Join batches with the same delivery policy and merge key, up to max_batches.