Add a focused transaction test for the branch where update extraction succeeds
but promoting `.local.installing` to `local` fails. The fake unpacker creates a
non-empty `local/` conflict after extraction, so the commit rename fails without
adding production hooks or brittle platform-specific permission tricks.
The assertion verifies the old install is restored from `.local.backup`, the
conflict and staging directories are removed, the backup is consumed, and the
intent is cleared back to None.
Test Plan:
- git diff --check
- just fmt
- just clippy
- just test
Follow-up-Plan: FOLLOW_UP_2.md
ddidderr
bb483f01f6