Bound libc DNS before falling back to Hickory so the manual
connector's two-second resolution budget cannot expire first.
Keep at most one system lookup in flight because Tokio cannot cancel
a blocking getaddrinfo call. This prevents reconnects from exhausting
the blocking pool while allowing the system resolver to recover.
Apply the policy to process-default and namespace-aware resolution,
and cover timeout, retry, and success behavior with deterministic
tests.
