test/format switch-mode stress aborts with either signature:

__rec_fill_tw_from_upd_select: "Consecutive tombstones found on the update chain"
__timestamp_no_ts_fix: 'stop_txn >= start_txn'

The trigger: a follower truncate carrying a read timestamp older than a committed ingest update misses that update while positioning its ingest cursors and writes no tombstone for it. Step-up's timestamp-ordered drain then stacks a tombstone on a tombstone.

The gap in WiredTiger is tracked in WT-17637, which is blocked on a decision doc. While that gap remains unplugged, we should update test/format to not run workloads that trigger it.

Change

Start serialized follower truncates without a read timestamp. This could be done in test/format directly, or as an API configuration option. Leader truncates and ordinary CRUD keep existing selection.