Two improvements from the live OSA upgrade run:
1. Boot environment rollback: step 2 (before any base changes) now creates
a BE with naming convention MAJOR.MINOR-upgrade-DD.mon.YY. pkgbase does
NOT auto-snapshot ZFS — this must be done manually. If the upgrade
misbehaves after reboot, bectl activate + reboot = instant rollback.
2. Cross-release override: pkgbase refuses to fetch 15.1 packages while
running 15.0 userland. Document the env OSVERSION=1501000 override
needed to cross the boundary.
Renumbered subsequent steps 2→3, 3→4, ..., 7→8.
