resolver: a root server's reply marked recursion available means interception (closes #300)

dnswatcher never asks for recursion and a root server never offers it,
so a reply marked recursion available from a root server's address
came from a resolver on the network answering in its place. Such a
reply is now passed over, and when every root server refused or gave
one, the lookup fails with the existing "this network intercepts DNS
queries", whether or not that resolver had the name cached. Before, a
cached name was checked against the resolver's second-hand answer.

On such a network dnswatcher and its tests fail with that error, by
design; nothing asks again another way or works around it. A live test
passes public resolvers as the root servers and expects the error.

Model: opus-5-5

README.md

@@ -561,6 +561,17 @@ This approach ensures:
 - Ability to detect split-horizon or inconsistent responses across authoritative
   servers.
 
+A root server never offers recursion, so a reply marked "recursion available"
+from a root server's address came from a recursive resolver on the network
+answering in its place, as on a network where a router rule, a VPN or a DNS
+filter sends all DNS to one resolver. Such a reply is passed over for the next
+root server. When every root server refuses a query or gives such a reply, the
+lookup fails with the error "this network intercepts DNS queries". Every lookup
+starts at the root servers, so on such a network every lookup fails with that
+error, whether or not that resolver has the name cached, and dnswatcher's tests
+fail there too. That is by design: nothing asks again another way or otherwise
+works around the interception.
+
 A watched name's records are stored as its nameservers return them, CNAME
 included. When they return a CNAME and no address, the chain of every CNAME
 target they gave is followed (with a depth limit to prevent loops) to the A and