plan: nextbsd-pkg install-test CI — never publish a repo that can't install itself (nextbsd#370)

Two-layer gate closing the userland<->compat file-conflict gap: a deterministic
cross-package ownership check at assembly time, plus a real host-side
pkg -r install of NextBSD-everything before publish (mirroring the ISO builders'
launchd guard + per-dep pkg info -e). Staged for review; not yet linked from index.

Author: pkgdemon