The err: block of __checkpoint_db_internal contains operations that involve cleaning up resources and resetting the checkpoint state.  This cleanup is a self-contained concern that doesn't need to be visible in the error path alongside tracking, rollback, and logging.

The aim of this ticket is to extract this block into a new static helper to improve readability and reduce code complexity.

Definition of Done

• _checkpoint_teardown exists as a static helper in checkpoint_txn.c and is called from _checkpoint_db_internal
• Existing tests pass