nextRetryAt

When the next automatic retry under spec is due (epoch milliseconds), as 9.x computes it: spec's minimum backoff, doubled per retry made, after the last retry started (after the failure, before the first one), and not before the last retry's claim expired (ParkedRetrySpec.claimTtl), so a retry that may still run on the partition's former owner is not started twice. Null when the entry is not retryable.