dnslist previously http-probed every wordlist candidate through the
blocking os resolver, so a big list meant a request per dead name and a
wildcard zone flooded results. resolve each candidate first via a new
internal/dnsx (retryabledns over a bundled 1.1.1.1/8.8.8.8/9.9.9.9 pool,
promoted to a direct dep), fingerprint the apex with random nonexistent
labels to detect catch-all zones, and http-probe only the names that
actually resolve and aren't wildcard. add -resolvers to override the
pool. resolverFn is a package-level seam so the dnsx tests stay
hermetic; the dnslist newDNSResolver seam keeps the integration test
network-free.
vmfunc