__checkpoint_db_internal contains four separate logging blocks scattered across the function - at the prepare, start, post-commit flush, and end stages. This scattering makes the function harder to scan and makes the overall checkpoint logging behavior harder to reason about.

The aim of this ticket is to extract all four logging blocks into a single static helper __checkpoint_log_stage, keyed by a stage enum.

Definition of Done

• __checkpoint_log_stage exists as a static helper in checkpoint_txn.c and is called from __checkpoint_db_internal at all four logging points
• Existing tests pass