A bootstrap lookup that returned only one address family, such as an
AAAA-only answer for a split-stack Control D upstream, was accepted as
is, and re-bootstrap rebuilt the transports from the same lists, so A
queries failed with "empty addresses" until a restart.
Bootstrap now checks the result against what the ip_stack needs and
fills a missing family from the Control D direct-IP table and bootstrap
server. If it is still missing, the next re-bootstrap looks it up again,
and a split transport borrows the other family meanwhile.
After the DoH direct-IP fallback fires, DoH and DoH3 also dial the
Control D direct IPs for the ip_stack's families when the bootstrap
addresses are empty or fail, keeping the domain for TLS. A working list
always wins, so nothing is pinned. DoT always verifies the server as
the upstream domain.
