Found N prepared transactions after mariabackup SST

Full message:
2026-06-28 15:12:19 0 [ERROR] Found 2 prepared transactions! It means that server was not shut down properly last time and critical recovery information (last binlog or tc.log file) was manually deleted after a crash. You have to start server with --tc-heuristic-recover switch to commit or rollback pending transactions.

Discovered when testing the fix for MDEV-38147:

On the joiner, the wsrep XID continuity check (handler.cc:wsrep_order_and_check_continuity()) recovers a contiguous run of prepared wsrep XIDs starting at the SE checkpoint and stops at the first gap, potentially leaving some prepared transactions uncommitted.
The post-recovery dry-run ha_recover(0) (mysqld.cc:init_server_conponents()) then finds those leftover prepared transactions and aborts.

The non-contiguous prepared set in the mariabackup snapshot is almost certainly the the consequence of wsrep_slave_threads>0: parallel appliers preparing out of order.
and the per-transaction rotation at 4096 makes the snapshot land mid-gap. It only affects the joiner.

log_bin=ON seems to be essential for reproduction