Issue 339's guard (#63/#64/#65) now refuses the machine's own trees, another account's
directory below a home, and uses a found directory as found; this branch's own copy of
those rules goes. What stays is the one rule only an account that never becomes root
needs: below its home it owns every parent and could swap a link in between the
engine's check and an unpack or a write-into (hq ADR 0266 §7). One reader of the user
database serves both.