part.met backup could be refreshed from the newly written metadata

Summary

The .part.met.bak file could be updated after the new .part.met was already promoted, making the backup a copy of the same bad metadata when the save cycle went wrong.

Mainline Outcome

Landed on main in commit 5aa4539 (BUG-053 preserve previous part met backup).

Backup creation now preserves the previous metadata before the atomic replacement of the live .part.met.