Function select_leading_session_command 

pub fn select_leading_session_command(candidates: &[ClaimCandidate]) -> usize

Select a leading session-command batch.

Returns 1 only when the earliest ready claimable batch is a QueuedWorkClass::SessionCommand. Session commands are intentionally claimed one batch at a time so the runtime applies each mutation and completion through its normal fenced commit path before moving to the next.