Before the core boundary this STEP backs up:
- VM100 lifecycle script and peer-state directory;
- VM121 agent source and env example.

Rollback restores those files and the previous selector/activity state, then
restores VM121 reconcile/watch services. The preflight requires runtime ==
VM121 desired and any registry extras absent from runtime, so the cutover does
not intentionally remove any live runtime peer before the core boundary.

After `CORE_CHANGE_COMPLETE=true`, late source/report/publication failures use
continuation and must not roll back a functioning lifecycle cutover.