Description

Fix a risk of checkpointing a partially applied binlog transaction. Multiple row changes can share the same binlog coordinates, so applying one does not prove that all have reached the ghost table. Resuming from such a checkpoint can skip unapplied changes.

• Send ordered transaction-completion markers through the existing reader, dispatcher, and apply queues.
• Advance a separate applied-transaction watermark only after preceding destination SQL commits. Persist that completed boundary rather than sampled reader or heartbeat progress.
• Preserve bounded queues and DML batching across markers. Failed SQL batches cannot acknowledge collected markers.
• Stop synchronous dispatch after listener errors and reject marker acknowledgement after abort. Make reader operations, checkpoint waits, and checkpoint SQL observe migration cancellation.
• Clone MySQL and MariaDB GTID coordinates so later mutations cannot change recorded transaction boundaries.

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.